rfc

This PR adds support for final class constants, as implemented in PHP 8.1.

example:

class Foo
{
    final public const X = "foo";
}

I also took the opportunity to break up the class_modifier node since it's inappropriately named with regard to its use and other similar constructs in the parser (e.g. static_modifier, var_modifer, visibility_modifier, etc.)

class_modifier has been replaced by abstract_modifier and final_modifier wrapped in a modifier-field.

Checklist:

• All tests pass in CI
• There are sufficient tests for the new fix/feature
• Grammar rules have not been renamed unless absolutely necessary (2 rules renamed - see explanation above)
• The conflicts section hasn't grown too much (0 new conflicts)
• The parser size hasn't grown too much (master: 1210, PR: 2116)