Adds option to always search/scan symlinks for more thorough coverage of
#41887 issue

This is a significant rewrite and more thoroughly reviewed and tested
version of the old PR here:
https://github.com/zed-industries/zed/pull/46344

The "never" option was removed from the old PR, since "expanded" is now
the default. Perhaps "never" can be added as an option later if there's
demand for it. Adding that option may resolve #48890 for instance.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable
(significant impact with "always" option, but "expanded" is default)

Release Notes:

- Add option choose between including expanded symlinks or include all
symlinks in project search (#41887)

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Co-authored-by: Eric Holk <eric@zed.dev>