Implement MSC4023 in matrix-js-sdk

[Johennes]

[Johennes]

@t3chguy looking at the diff in matrix-org/matrix-js-sdk#3427, I think this should actually already be covered by that PR?

[t3chguy]

@Johennes yes that PR implements the unstable identifiers for the MSC, along with a fallback to handle (at a perf hit) compatibility with servers lacking that feature (all of them right now)