Feature: sway/window can show 'shell' parameter

[TheRealLorenz]

This is a very simple pull request, which grants the sway/window module the capability to display the 'shell' parameter from sway tree.

I personally find it very handy whenever I need to check if a certain window is running natively on Wayland or not.

I was wondering if I should also edit the manpage file for sway/window and the wiki.

Please feel free to correct my mistakes and formatting.

[Alexays]

LGTM, and yes if you can update the man <3

[TheRealLorenz]

I'm not entirely sure about what should I put as default settings, btw.

Should I also update the wiki?

[Alexays]

Yes if you can also update the wiki that would be perfect

[TheRealLorenz]

Yes if you can also update the wiki that would be perfect

Done!