Create decky-updater.sh

[brendenhoffman]

This is an interactive script to install or update decky-loader.

I wrote this script since decky-loader often can't be updated easily through the menu after it is broken by a SteamOS update. Not entirely necessary, but I personally prefer to run an existing script that I have installed rather than using curl to run a specific script. Not sure the best way to obtain this specific script the first time in a way that is easy for the Steam Deck crowd, besides pulling it along with decky-loader, but its something I wanted to put out there anyway.

[PartyWumpus]

Looks good. I think being able to uninstall from it makes sense, so I'm going to add it. If you have a reason why not, feel free to revert my change.