Better implement collaborator presence & prevent flickering

[doc-han]

User story

When a user is on the collaborative editor with other users, we want to

1. know exactly what state the other users are in whether active|away|idle
2. when users leave their tab to another tab, there's a change JS execution in the tab gets throttled which makes these users flicker. we need to resolve that.

Details

• [a] Initial solution for this has been implemented via fix: flickering of active collaborator icons between states #3951 which caches all remote users to prevent the flickering. It also makes use of the lastSeen variable to determine the activity state of a user.
• [b] There's another PR fix: flickering of inactive collaborator #4053 which uses a different method, two states, 1. lastSeen to keep updating ydoc and 2. lastState to determine the actual state of a user. It also solves flickering by only caching away users. This is because we can assert that, throttling is likely to only happen for away users.

Implementation notes

Since [a] has already been merged into main. there doesn't seem to be much conflict from [b] to main tho. but we need to make few changes to make sure [b] can land in main safe and beautifully.

1. do we still need the caching at the useAwareness level? this is because [b] puts the caching at the useRemoteUsers level. useRemoteUsers is just like useAwareness but shows a single user for their several connections sorted by the active connection then lastSeen.

2. rebase [b] onto main

3. resolve the difference in tests between [b] and [a]

Release notes

User acceptance criteria