Allow alternative keybindings

**Is your feature request related to a problem? Please describe.**
My Magit muscle memory and ergonomic awareness tell me to use `n`/`p`/`c-n`/`n-p` for up/down navigation instead of reaching for arrow keys whenever I'm alternating between typing and navigating.

**Describe the solution you'd like**
Allow each action to have multiple keybindings.

Example:
```yaml
prevItem: [ , ]
nextItem: [ , ]
```

**Describe alternatives you've considered**
- give up arrow keys
- (but I often alternate between Emacs and arrow cursor navigation)
- just use arrow keys
- (but it's un-ergonomic)
- look for a terminal app wrapper which can remap keys for just one app
- (but it would be a hack, and I also need this for both Linux and Windows)
- look for an alternative to lazygit
- (but lazygit is great and I haven't found a better one – Magit is prohibitively slow on Windows, gitui lacks some features)