CI: Introduce PHP compatibility check

[Wirone]

Inspired by this finding, bug introduced in #7616, reproduced here - we have invalid return type in code and none of the tools caught this. I thought this is because we run PHPStan on 8.3 which then assumes it's valid return type, but I modified dev-tools/composer.json, allowed PHP 7.4 and checked on it, but PHPStan still did not report it, even on max level. I believe this is a bug (CC: @ondrejmirtes), but anyway I thought maybe it's a good idea to introduce PHPCompatibility check, which is more focused on differences between PHP versions.

This is a draft and CI will fail, because we need to decide how we want to handle dynamic usages of PHP consts etc. I believe that sniffs should not report these problems if constant is used in if condition, like:

if (\defined('T_READONLY')) {
    // Skip readonly classes for PHP 8.2+
    $acceptTypes[] = T_READONLY;
}

but this is separate problem that should be addressed and fixed in long-term (CC: @jrfnl).

PS. There is 10.x-dev as 9.99.0 constraint because 10.x development version has more checks, but at the same time is not compatible with phpcompatibility/phpcompatibility-symfony which handles Symfony polyfills. We've been using this setup for months in GetResponse and it helped us with migration from 7.4 to 8.2, so I assume it as safe enough 🙂.

[coveralls]

coverage: 94.769%. remained the same
when pulling 9cdb005 on Wirone:codito/phpcompatibility
into ea01071 on PHP-CS-Fixer:master.

[ondrejmirtes]

Please don't cc me on other repositories. If you want, you can isolate the bug, reproduce it and report it properly on PHPStan's issue tracker.

[julienfalque]

I would include the main file too:

Suggested change

	<file>../../src/</file>
	<file>../../tests/</file>
	<file>../../src/</file>
	<file>../../tests/</file>
	<file>../../php-cs-fixer</file>