Reuse workspace on new journal entry command if possible (#16924)

Closes #6783

With this PR, the `journal: new journal entry` command only opens a new
workspace if the current workspace does not already contain the
`journal` directory. Both the root of the work tree and all its
subdirectories are checked.

This does not yet check for the day's file specifically, as suggested
[here](https://github.com/zed-industries/zed/issues/6783#issuecomment-2268509463).

I'm new to writing Rust code in production (as well as contributing in
general), so any feedback is much appreciated!

Release Notes:

- Reuse workspace on `journal: new journal entry` command if possible