gum choose: Keybindings to invert selection

[lwbt]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
I'd like to have keybindings to invert (e.g. i) a selection in gum choose and to have these keybindings documented in a way that I don't need to read the source code itself (requiring understanding of how the code works).

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
#388 added the shortcut to select all or none, but my FR for a shortcut to invert was overlooked.