This PR adds tracking of project groups to the MultiWorkspace and
serialization/restoration of them. This will later be used by the
sidebar to provide reliable reloading of threads across Zed reloads.

Release Notes:

- N/A

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Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>