Closes #49957 

Also adds `start_of_input` context, and modifies both
`{start,end}_of_input` to work for both single line and auto height
editor modes.


https://github.com/user-attachments/assets/e30f2b20-a96c-49d5-9eb6-3c95a485d14a

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:

- Added support for multi-line search and replace input in Buffer Search
and Project Search

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Co-authored-by: Nathan Sobo <nathan@zed.dev>