Add support for browser.webRequest API

[RickyMarou]

There is no implementation at all for the browser.webRequest API.

Also since this API will be/is going through changes with MV3 (see here), it'll be awesome to be able to configure the mocking to be MV2/MV3 so we can write unit tests for these different scenarios.

[goatonabicycle]

This would definitely be awesome!