Implement readonly modifier for classes

[thekid]

See https://wiki.php.net/rfc/readonly_classes and xp-framework/ast#37. The following comes quite close to records, although it doesn't declare getters but uses public readonly fields:

readonly class Point {
  public function __construct(
    public int $x,
    public int $y,
  ) { }
}

$point= new Point(8, 42);
echo "Hello {$point->x}x{$point->y}!\n";

// Will not modify the member, raising an exception instead:
// $point->x= 9;

• Inherit readonly modifiers on classes to all members and promoted constructor parameters
• Prevent dynamic members
• Do not allow readonly on traits, interfaces and enums
• Prevent inheritance where readonly modifiers do not match
• Prevent static members in readonly classes
• Disallow #[AllowDynamicProperties] on readonly classes
• Remove emulation for PHP 8.2 once Add support for readonly classes php/php-src#7305 is merged

[thekid]

This proper error handling means any way of raising an exception from within the emitting phase and having it turned into a lang.ast.Error with file and line set.

[thekid]

Maybe these also need to go into the parser - where, instead of returning lang.ast.Node instances directly, we'd do something like:

// Before
return new PropertyNode(...);

// After
return $this->ast->property(...);

...and have two separate ast implementations: checked and unchecked. This would be part of a greater refactoring, thus keeping it for a separate pull request.

[thekid]

Released in https://github.com/xp-framework/compiler/releases/tag/v8.4.0 - removing the emulation from PHP 8.2 emitter can be done once the php-src PR is merged, and released in a separate release.