Move URL out of RequestInfo to match DOM types

[penalosa]

Fixes #3090

To improve the typing situation when loading dom types in the same environment as workers-types, this PR moves the URL type out of RequestInfo, in favour of e.g. fetch() referencing fetch(req: RequestInfo | URL) directly. This matches how dom types are defined.

There should be no user-observable behaviour change for users using just workers-types, but this should improve the situation if e.g. the fetch definition comes from workers-types but the RequestInfo definition comes from dom types.

[petebacondarwin]

I manually applied this fix (or approximately this fix 😄) to the reproduction in the issue and it appears to resolve the typings problem.

[andyjessop]

Nice work 👍