new URL() constructor does not take URL object as base parameter

[Nullizer]

https://url.spec.whatwg.org/#example-5434421b

A URL object can be used as base URL (while IDL requires a string as argument, a URL object stringifies to its href attribute value):
var url = new URL("🏳️‍🌈", new URL("https://pride.example/hello-world"))

Now it's only support DOMString as base parameter, need explicit call toString method on base to avoid compile error.

Related: microsoft/TypeScript#15246

[Nullizer]

I made a PR for this #332

[saschanaz]

Ideally we should support interfaces with stringifiers, not sure how.