Support for enums and PHP constants constants in YAML files

Drupal now supports PHP constants in YAML files. For example:

parameters:
    # this is considered a regular string
    foo: PHP_INT_MAX
    # this is considered a PHP constant
    bar: !php/const PHP_INT_MAX

Will be read as

[
    "parameters" => [
      "foo" => "PHP_INT_MAX",
      "bar" => 9223372036854775807,
    ],
]

This also allows us to support enums in config and service yamls too. For example:

enum Suit {
    case Hearts;
    case Diamonds;
    case Clubs;
    case Spades;
}

$data = [
  'suit' => Suit::Hearts
];
dump(\Drupal\Component\Serialization\Yaml::encode($data));

Produces the following YAML:

suit: !php/const Suit::Hearts