Parametrize paths (tmux 3.2/3+?), add navigation toggle

[deliciouslytyped]

This PR enables using nested-tmux without hardcoded paths, once the two patches in https://github.com/tmux/tmux/issues/2638 and https://github.com/tmux/tmux/issues/2637 land in upstream tmux.

It also makes the state machine more complicated, by allowing you to toggle the M-arrow navigation and the prefix with M-c (the binding is arbitrary).

With this, you can now easily test nested-tmux with a command like tmux -f plugin.conf -L test.

(Yes, I really should have used multiple commits for this.)

[rpatterson]

I'm not sure but you might find what I do with parameterized paths in my TPM plugin PR relevant to what your trying to accomplish.