feat(tpm): Convert to a TPM plugin

[rpatterson]

Add the boilerplate necessary to use this project as a TPM plugin. Works for me locally using set -g @plugin 'file:///usr/local/src/nested-tmux' and symlinking that into ~/.tmux/plugins/. IOW, I haven't tested this end-to-end but I'm not aware of any reason it shouldn't work.