This adds very basic cache (literally just a `Map` for now) to store room alias
to room ID mappings. The improves the perceived performance of Riot when
switching rooms via browser navigation (back / forward), as we no longer try to
resolve the room alias every time.
The cache is only in memory, so reloading manually or as part of the clear cache
process will start afresh.
Fixes https://github.com/vector-im/riot-web/issues/10020
