Use URL instance in "req.url"

[kettanaito]

This suggests a breaking change.

Is your feature request related to a problem? Please describe.
I suggest to store an intercepted request's URL as a URL instance, as opposed to string.

Benefits

• URL class is supported in both browser and Node environments;
• Allows to unify internal implementation in the underlying libraries, leading to fewer URL transformations;
• Gives more information to the developer, such as req.url.protocol, req.url.port, etc. This information is already available, just not exposed to the developer, which is an experience flaw;

Additional context
There are multiple points of conversion happening, which transform a string representation of a request's URL into a URL instance anyway:

• node-request-interceptor converts it to URL in order to provide a clean URL in the middleware function;
• msw@getResponse converts to a URL instance to get a clean URL. With Supports running mocks in Node #146 it will also use that URL instance to resolve a relative URL against the current window.location.