Follow-up of https://github.com/zed-industries/zed/pull/31457

Add a button and also allows to use `search::ToggleIncludeIgnored`
action in the file finder to toggle whether to show gitignored files or
not.
By default, returns back to the gitignored treatment before the PR
above.


![image](https://github.com/user-attachments/assets/c3117488-9c51-4b34-b630-42098fe14b4d)


Release Notes:

- Improved file finder to include indexed gitignored files in its search
results