Add shellcheck

[daPhipz]

I'm being completely honest with you all - I have not very much experience with coding and stuff, but I already wrote shell scripts myself and "sort of" can say what this script is doing from looking at the code.

Looking at your desktopify project, I noticed someone implementing automated shellcheck checks for every PR via GitHub Actions.
Do we want something similar here as well?

I will attempt to fix all the warnings and errors that the current version of quickemu will throw at me as well.