Move interfaces defined by the WebRTC specification to the
`script/dom/webrtc/` module from `script/dom/`.
`script/dom/rtc*.rs -> script/dom/webrtc/`
Testing: No changes, just a refactoring
Fixes (partially): #38901
Signed-off-by: Andrei Volykhin <volykhin.andrei@huawei.com>
Co-authored-by: Andrei Volykhin <volykhin.andrei@huawei.com>