- add Linux/Windows keybindings
- use consistent spacing around the `+` in key combos

Release Notes:

- I don't have access to a Mac, so I haven't verified any of the macOS
keybindings. I suspect some may be out of date. Please verify.
- All other migration guides should be updated with Linux/Windows
shortcuts, and should use consistent spacing around the `+`:
  - https://zed.dev/docs/migrate/vs-code#differences-in-keybindings
  - https://zed.dev/docs/migrate/intellij#differences-in-keybindings
  - https://zed.dev/docs/migrate/pycharm#differences-in-keybindings
  - https://zed.dev/docs/migrate/rustrover#differences-in-keybindings

---------

Co-authored-by: MrSubidubi <finn@zed.dev>