Support MSC4222 state_after (#4487)

* WIP support for state_after

* Fix sliding sync sdk / embedded tests

* Allow both state & state_after to be undefined

Since it must have allowed state to be undefined previously: the test
had it as such.

* Fix limited sync handling

* Need to use state_after being undefined

if state can be undefined anyway

* Make sliding sync sdk tests pass

* Remove deprecated interfaces & backwards-compat code

* Remove useless assignment

* Use updates unstable prefix

* Clarify docs

* Remove additional semi-backwards compatible overload

* Update unstable prefixes

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Fix test

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add test for MSC4222 behaviour

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Improve coverage

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Fix tests

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Tidy

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add comments to explain why things work as they are.

* Fix sync accumulator for state_after sync handling

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add tests

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Revert "Fix room state being updated with old (now overwritten) state and emitting for those updates. (#4242)"

This reverts commit 957329b218.

* Fix Sync Accumulator toJSON putting start timeline state in state_after field

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Update tests

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add test case

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

---------

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>
Co-authored-by: Hugh Nimmo-Smith <hughns@matrix.org>
Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>
Co-authored-by: Timo <toger5@hotmail.de>

spec/test-utils/test-utils.ts

@@ -86,7 +86,7 @@ export function getSyncResponse(roomMembers: string[], roomId = TEST_ROOM_ID): I
     };
 
     for (let i = 0; i < roomMembers.length; i++) {
-        roomResponse.state.events.push(
+        roomResponse.state!.events.push(
             mkMembershipCustom({
                 membership: KnownMembership.Join,
                 sender: roomMembers[i],