Add support for single-expression methods

[thekid]

This PR adds syntactic support for single-expression methods

class Person {
  public function __construct(private string $name) { }

  // Equivalent of public function name() { return $this->name; }
  public function name() => $this->name;
}

See:

• https://wiki.php.net/rfc/single-expression-functions
• https://github.com/xp-lang/php-compact-methods (same but uses fn)
• feat: single expression functions php/php-src#17677