This PR factors some more code related to the `RecentBuffersContext` out
of the `AssistantPanel` and into the corresponding module.

We're trying to strike a balance between keeping this code easy to
evolve as we work on the Assistant, while also having some semblance of
separation/structure.

This also adds the missing functionality of updating the remaining token
count when the `CurrentProjectContext` is enabled/disabled.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>