Render `+` separators for keybindings on non-macOS platforms (#9194)

This PR adjusts the rendering of keybindings on non-macOS platforms to
have a `+` separator instead of just a blank space.

<img width="952" alt="Screenshot 2024-03-11 at 3 18 17 PM"
src="https://github.com/zed-industries/zed/assets/1486634/1573823d-4329-41f0-bef4-7a6c09f3e632">

<img width="584" alt="Screenshot 2024-03-11 at 3 16 25 PM"
src="https://github.com/zed-industries/zed/assets/1486634/aae41b22-dfde-40a6-9e0e-cee855522d3a">

Release Notes:

- N/A