Mainline GPUI2 UI work (#3079)

This PR mainlines the current state of new GPUI2-based UI from the
`gpui2-ui` branch.

Release Notes:

- N/A

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Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Nate <nate@zed.dev>