migrator: In-memory migration and improved UX (#24621)

This PR adds:

- Support for deprecated keymap and settings (In-memory migration)
- Migration prompt only shown in `settings.json` / `keymap.json`.

Release Notes:

- The migration banner will only appear in `settings.json` and
`keymap.json` if you have deprecated settings or keybindings, allowing
you to migrate them to work with the new version on Zed.