9.3.0: PHP 8.4 emitter

• Fixed checks for property hooks emulation with asymmetric visibility
  (@thekid)
• Added PHP 8.4 emitter which natively emits property hooks and asymmetric
  visibility syntax. This is integration-tested with PHP 8.4.0 Beta 4.
  See https://github.com/php/php-src/blob/php-8.4.0beta4/NEWS
  (@thekid)
• Changed emitter to use native readonly classes in PHP 8.2, fixing an
  inconsistency with accessing undefined properties
  (@thekid)