Merge `quick_action_bar` into `zed` (#21026)

This PR merges the `quick_action_bar` crate into the `zed` crate.

We weren't really gaining anything by having it be a separate crate, and
it was introducing an additional step in the dependency graph that was
getting in the way.

It's only ~850 LOC, so the impact on the compilation speed of the `zed`
crate itself is negligible.

Release Notes:

- N/A