Throttle the sending of UpdateFollowers messages (#8918)

## Problem

We're trying to figure out why we sometimes see high latency when
collaborating, even though the collab server logs indicate that messages
are not taking long to process.

We think that high volumes of certain types of messages, including
`UpdateFollowers` may cause a lot of messages to queue up, causing
delays before collab sees certain messages.

## Fix

This PR reduces the number of `UpdateFollowers` messages that clients
send to collab when scrolling around or moving the cursor, using a
time-based throttle.

The downside of this change is that scrolling will not be as smooth when
following someone. The advantage is that it will be much easier to keep
up with the stream of updates, since they will be sent much less
frequently.

## Release Notes:

- Fixed slowness that could occur when collaborating due to excessive
messages being sent to support following.

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Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>