Implement property hooks syntax

[thekid]

This pull request adds syntactic support for property hooks.

• Braced forms: get, set { <statements> }
• Abbreviated forms: get, set => <expr>
• Abbreviated getter: $prop => <expr>
• Specifying the argument name: set($value)
• Specifying the argument type: set(string $value)
• Final hooks: final get|set
• Abstract hooks set; get; (see here)
• References - see https://externals.io/message/120213#120362

class Person {
  public $modified= false;

  public $name {
    set {
      $this->name= $value;
      $this->modified= true;
    }
  }
}

$p= new Person();
$p->name= 'Test';
$p->modified; // true

See https://wiki.php.net/rfc/property-hooks

[thekid]

We are not adding this as constructor parameter partly in order to prevent a BC break, partly because of how parsing works.

[thekid]

The RFC has been accepted, however php/php-src#13455 isn't merged at the time of writing, so we don't have a reference implementation quite ready yet for testing this thorougly.