[WIP] Replace in project (#2984)

Targeting Preview of 09.27.
This is still pending several touchups/clearups:
- We should watch multibuffer for changes and rescan the excerpts. This
should also update match count.
- Closing editor while multibuffer with 100's of changed files is open
leads to us prompting for save once per each file in the multibuffer.
One could in theory save in multibuffer before closing it (thus avoiding
unnecessary prompts), but it'd be cool to be able to "Save all"/"Discard
All".

Release Notes:

- Added "Replace in project" functionality