mount.yazi: Show a "legend" with the available keybindings and actions

[kankaristo]

It would be nice if there was a legend at the bottom (or top) of the mount.yazi window with the available keybindings and actions.

The keybindings are not that difficult to remember, "m" to mount, "u" to unmount, and "e" to eject. But, I often go a couple of months between using a USB drive, and I might not remember that there are separate keybindings/actions for unmount and eject

And in case more features are added (like renaming a drive label, formatting a drive, etc.), the user would automatically find out about the new feature by seeing the new keybinding+action appearing in the legend, without reading release notes or re-reading the readme.

I realize that this might be a lot of work for a small feature, since I guess the legend's visibility should be configurable (you might not want the legend in a small terminal), you'd have to actually get the keybindings (in case they're not set to the defaults), and I'm not sure if there's an existing interface component in Yazi that supports this. But a boy can wish.

[sxyazi]

Duplicate of #132