feat(php): Support literal types

[ajgateno]

No description provided.

[amckinney]

Discussed offline w/ @ajgateno - with the amount of one-off handling we need to accommodate setting literals under the hood, this feels bug prone and easy to mess up.

To start, we'll take on a design similar to the literal experience in TypeScript - strongly type each call-site in PHPDoc comments, but still require the user to specify them by hand in the constructor. We can always improve this experience later on if there's enough reason to, but this feels like the right middle ground to start.

[amckinney]

Few nits + updates in the error messages we throw.

[amckinney]

Looks good - some of the large unions have really large PHPDoc comments, but it feels safer to start in the more strict manner, and relax that requirement later on (if ever).