bug#76854: Add tmux-sessionizer plug to Guix

[Steve George]

Hi Alex,

Thanks for contributing this package. I added them to the rust-team branch 
which will fold into master eventually:

https://git.savannah.gnu.org/cgit/guix.git/commit/?h=rust-team&id=99d1e68a9558a35018b22b21755c14e9518d6788

These were all pretty clean to apply, thank-you! One note for your commit 
messages the format is:

gnu: [Add|Update] <variable>

*/path/to/file (variable): [detail of change]

I think you weren't aware of the second line where you have to have the file 
path and the change. Anyway, no big deal - thanks for your efforts!

Futurile / Steve