If pedantic_wpt is set, strictly validate Request fields even if not implemented

Even if we don't implement certain request feartures, we should still validate that the passed options follow the standard:

For RequestInit specifically:
- RequestInit's window is not null
- Input URL has credentials
- RequestInit's mode is navigate
- RequestInit's referrer is invalid
- RequestInit's method is forbidden
- RequestInit's mode is no-cors and method is not simple
- Bad referrerPolicy init parameter value
- Bad mode init parameter value
- Bad credentials init parameter value
- Request with cache mode: only-if-cached and fetch mode: same-origin

In general:
- Check destination attribute
- Check referrer attribute
- Check referrerPolicy attribute
- Check mode attribute
- Check credentials attribute
- Check cache attribute
- Check isReloadNavigation attribute
- Check isHistoryNavigation attribute
- Check duplex attribute