Note: There is still some consistency work to do between all the
keybinding icons. I'll tackle this at a later time.

- Adds `return`, `space`, `tab`, `escape`, `pgup` and `pgdn`
- Simplifies keybinding rendering (due to theme changes)
- Standardizes icon naming convention to use `_` everywhere.

Release Notes:

- Zed 2: Added additional keybinding icons