Support reading keyboard input

[jemag]

This has come up a few times in various issues such as:
#1807
#1962

and numerous times discussing Zellij with peers and colleagues. A lot of users are currently using tmux and not being able to use their keybinds within Zellij because it only supports STDIN is a big blocker to transition.

I realize that there are some use cases where the behavior may not be consistent (ssh), however, nowadays most developers and general users will spend the vast majority of their time on their local setup.

I truly believe that this would improve the experience of the vast majority of users in most use cases. This would also be a more intuitive experience for new users of Zellij, which seems to be something that the project values.

[imsnif]

Honestly, if someone wants to implement it in a way that Zellij will accept both (and I assume there's some magic mechanism from the various window managers that will mean keyboard input will only come from the focused window), I'd accept this PR.

Of course with the usual caveats of cross platform compatibility, and a warning in the logs when "keyboard-only" keys are defined, so that users can know why spamming certain keys over SSH isn't working for them.