This fixes a deadlock that would occur when `DapStore` had its on quit
handler called. The deadlock was caused by `DapStore` spawning on the
main thread while `App::shutdown` blocks the main thread.

We added a debug_panic in GPUI that panics if a foreground task is
spawned while the App context is shutting down. This will help tests
catch hangs in `cx.on_app_quit` calls.

Release Notes:

- N/A

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>