PHP 8.3 support and various fixes

[calebdw]

Checklist

• All tests pass in CI
• There are enough tests for the new fix/feature
• Grammar rules have not been renamed unless absolutely necessary (0 rules renamed)
• The conflicts section hasn't grown too much ( 1 new conflicts)
• The parser size hasn't grown too much (master: 2678, PR: 2681)
  (check the value of STATE_COUNT in src/parser.c)

Hello, this combines #200, #206, and #207 at the advice from @amaanq.

• feat: add support for dynamic class constant fetch
• fix: single types being incorrectly parsed as a union_type
• fix: comment precedence

Closes #156, closes #197

Thanks!