Add a way to filter items in the outline panel (#13984)

https://github.com/zed-industries/zed/assets/2690773/145a7cf2-332c-46c9-ab2f-42a77504f54f

Adds a way to filter entries in the outline panel, by showing all
entries (even if their parents were collapsed) that fuzzy match a given
query.

Release Notes:

- Added a way to filter items in the outline panel