[CI] Add ShellCheck workflow to GitHub Actions

Summary: Relevant Linear issue here. Used the example here as inspiration for the YAML file.

Test Plan: Can probably land and see what happens when it first runs, judging off discussion @ashoat and @atul had in D4239.

Reviewers: atul

Reviewed By: atul

Subscribers: tomek, adrian, ashoat, atul

Differential Revision: https://phab.comm.dev/D4755