This fixes "Error compiling regex: Look-behind assertion without
constant size" when using variable-length lookbehinds in search, such as
`(?<=^(?:\s{4})*)\s{4}`.

fancy-regex 0.17.0 adds support for variable-length lookbehinds using
regex-automata's DFA reverse search commit:
https://github.com/fancy-regex/fancy-regex/commit/b58fb31

Closes #50219 

Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Fixed the error message on compilation of regular expressions
containing variable length lookbehind assertions

Previously:
<img width="1582" height="1377" alt="image"
src="https://github.com/user-attachments/assets/3e275891-aef5-482e-87f0-c47b9eac3c2a"
/>


Now:
<img width="1323" height="1015" alt="image"
src="https://github.com/user-attachments/assets/8638cd08-fcd0-40f9-8749-32e6352e89b7"
/>


Thanks to @vorahardik7 for helping out with a test example and
@AmaanBilwar for suggesting a fix!

Signed-off-by: Pratik Karki <pratik@prertik.com>