Prefix the user+device state key if needed (#4262)

* Prefix the user+device state key if needed

No need to prefix it for rooms that use MSC3779.
Otherwise, prefix it to bypass the auth rule for state events with keys
starting with @.

* Use RegExp.exec() method instead

Sonar typescript:S6594

* Split nested ternary operator into method

Sonar typescript:S3358

* Add test coverage

spec/unit/matrixrtc/mocks.ts

@@ -30,6 +30,7 @@ export function makeMockRoom(membershipData: MembershipData, localAge: number |
         getLiveTimeline: jest.fn().mockReturnValue({
             getState: jest.fn().mockReturnValue(roomState),
         }),
+        getVersion: jest.fn().mockReturnValue("default"),
     } as unknown as Room;
 }