Improve type safety of URLPattern args

[nickserv]

This makes the types closer to the behavior of the polyfill and the Chrome implementation by using a discriminated union to determine if the second arg is allowed, and then sharing the args between methods in a tuple type.

Note that this is a breaking change for TypeScript users, as I wanted to get rid of URLPatternInput to improve type safety. If possible, I'd recommend a major release for this.

[kenchris]

@SanderElias can you review this?