…tructor is **NOT** immutable

`DateTimeImmutable` is **almost** immutable: `DateTimeImmutable::__construct()` is in fact not a pure
method, since `new DateTimeImmutable('now')` produces a different value at each instantiation (by design).

This change makes sure that `DateTimeImmutable` loses its `@psalm-immutable` class-level marker,
preventing downstream misuse of the constructor inside otherwise referentially transparent code.

Note: only pure methods are stubbed here: all other methods declared by `DateTimeImmutable` (parent interface)
are NOT present here, and are either inferred from runtime reflection, or `CallMap*.php` definitions.

Methods are sorted in the order defined by reflection on PHP 8.1.8, at the time of writing this ( https://3v4l.org/3TGg8 ).

Following simplistic snippet was used to infer the current signature:

```php
<?php

$c = new \ReflectionClass(\DateTimeImmutable::class);

$methods = array_map(function ($m) {
    return $m->getName()
        . '(' . implode(',', array_map(function ($p) {
            return $p->getType()
                . ' $' . $p->getName()
                . ($p->isOptional() ? ' = ' . var_export($p->getDefaultValue(), true) : '');
        }, $m->getParameters())) . ')' . ($m->getReturnType() ? (': ' . $m->getReturnType()) : '');
}, $c->getMethods());

$properties = array_map(function ($m) {
    return $m->getName();
}, $c->getProperties());

var_dump($methods, $properties);
```

…mutable`

`DateTimeImmutable` is not really immutable, therefore this marker was wrong upfront

…ever `false`

…to use

Ref: https://github.com/vimeo/psalm/pull/8350/files/c205d652d1e9afd9510db59e72c3fd0a4a093b3d#r934032422

The idea is that `@psalm-pure` disallows `$this` usage in child classes,
which is not wanted, while `@psalm-mutation-free` allows it.

By using `@psalm-mutation-free`, we don't completely destroy inheritance
use-cases based on internal (immutable) state.

…red by stub

… stub

…ly covered by stub

… by stub

Note: some conditional return type magic was required here.

See: vimeo#8350 (comment)

…vered by stub

Note: also verified that a `DateTimeImmutable#getTimezone()` always returns
a default timezone (initialized internally), and therefore restricted the
type a bit.

…red by stub

…overed by stub

This also simplifies the return type from `int|false` to always `int`,
since a timestamp can always be produced.

Ref: https://github.com/php/php-src/blob/eff9aed1592f59cddb12d36a55dec0ccc3bbbfd6/ext/date/php_date.stub.php#L496-L500

…y stub

… by stub

Also expanded the return type from `static` to `static|false`, since the
operation can fail (with a warning too), such as in following example:

https://3v4l.org/Xrjlc

```php
<?php

var_dump(
    (new DateTimeImmutable())
        ->modify('potato')
);
```

Produces

```
Warning: DateTimeImmutable::modify(): Failed to parse time string (potato) at position 0 (p): The timezone could not be found in the database in /in/Xrjlc on line 6
bool(false)
```

Ref: https://github.com/php/php-src/blob/534127d3b22b193ffb9511c4447584f0d2bd4e24/ext/date/php_date.stub.php#L508-L509

… stub

Ref: https://github.com/php/php-src/blob/534127d3b22b193ffb9511c4447584f0d2bd4e24/ext/date/php_date.stub.php#L511-L512
Ref: https://github.com/php/php-src/blob/534127d3b22b193ffb9511c4447584f0d2bd4e24/ext/date/php_date.c#L3129-L3144

…vered by stub

Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.

On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.

Ref (PHP 8.2.x): https://github.com/php/php-src/blob/534127d3b22b193ffb9511c4447584f0d2bd4e24/ext/date/php_date.c#L3291-L3307
Ref (PHP 8.2.x): https://github.com/php/php-src/blob/534127d3b22b193ffb9511c4447584f0d2bd4e24/ext/date/php_date.stub.php#L517-L518
Ref (PHP 7.0.33): https://github.com/php/php-src/blob/bf574c2b67a1f786e36cf679f41b758b973a82c4/ext/date/php_date.c#L3363-L3379

…d by stub

Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.

On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.

Ref (PHP 8.1.x): https://github.com/php/php-src/blob/32d55f74229e7913db0d59ef874a401744479b6a/ext/date/php_date.c#L3212-L3228
Ref (PHP 7.0.33): https://github.com/php/php-src/blob/bf574c2b67a1f786e36cf679f41b758b973a82c4/ext/date/php_date.c#L3447-L3463

…d by stub

Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.

On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.

Ref (PHP 8.1.x): https://github.com/php/php-src/blob/32d55f74229e7913db0d59ef874a401744479b6a/ext/date/php_date.c#L3258-L3274
Ref (PHP 7.0.33): https://github.com/php/php-src/blob/bf574c2b67a1f786e36cf679f41b758b973a82c4/ext/date/php_date.c#L3496-L3512

…ered by stub

Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.

On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.

Ref (PHP 8.1.x): https://github.com/php/php-src/blob/32d55f74229e7913db0d59ef874a401744479b6a/ext/date/php_date.c#L3308-L3324
Ref (PHP 7.0.33): https://github.com/php/php-src/blob/bf574c2b67a1f786e36cf679f41b758b973a82c4/ext/date/php_date.c#L3549-L3565

…overed by stub

Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.

On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.

Ref (PHP 8.1.x): https://github.com/php/php-src/blob/32d55f74229e7913db0d59ef874a401744479b6a/ext/date/php_date.c#L3353-L3369
Ref (PHP 7.0.33): https://github.com/php/php-src/blob/bf574c2b67a1f786e36cf679f41b758b973a82c4/ext/date/php_date.c#L3596-L3612

…lly covered by stub

Also:

 * a non-empty format string will always lead to `non-empty-string`
 * it seems that you can throw **everything** at `DateTimeInterface#format()`, even null bytes,
   and it will always produce a `string`

While there is value in declaring `DateTimeImmutable::createFromInterface()` as mutation-free in
a stub, this method was introduced in PHP 8.0, so we cannot really declare it in a stub.

For now, we drop it, as the value of its stub declaration is much lower than the problems it
introduces through its conditional existence.