Add support for PHP 8s nullsafe operator.

[j-applese3d]

I'm not sure that I got everything right, so please review and let me know what I need to change :)

For instance the test will fail if you run it with PHP <8.0, and I wasn't certain that specific test file was the right one to update or if I should make a new one. I didn't know where to put it...

Thanks!
Related Issue: #633

[wisskid]

@j-applese3d is there any way to compile this into PHP7-compatible PHP code? I.e. to implement the nullsafe operator in Smarty without actually using the nullsafe operator in the compiled code? If not, we'll have to drop support for PHP7.x and we cannot release this until v6.

[j-applese3d]

@wisskid I believe that there are 2 different ways ?-> can be used:

1. to read an object property
2. to call a method

Both of these cases should be easy enough to implement in PHP compatible with 7.0.
Here are some example functions:

/** {$obj?->doSomething($p, $p2)} ➡ nullSafeMethod($obj, "doSomething", [$p, $p2]); */
function nullSafeMethod($object, $function, ...$params)
{
    if ($object === null) {
        return null;
    }
    return $object->{$function}($params);
}

/** {$obj?->someProperty} ➡ nullSafeProperty($obj, "someProperty"); */
function nullSafeProperty($object, $property)
{
    if ($object === null) {
        return null;
    }
    return $object->{$property};
}