Add ShellCheck and fix many warnings

[lorenzleutgeb]

Closes #105

1. I decided to ignore three errors for now:
   • SC2094 only occurs once. I do indeed think it is a problem that git ignore will read and write to .gitignore in one chain of pipes. However, I do think that the alias should be rewritten (use git status -u).
   • SC2119 and SC2120 go together, and they complain about the !f() { ... }; f gadget that many aliases use.
2. Created a workflow for GitHub Actions. Not sure whether shellcheck.sh really belongs into /, but I couldn't think of a better place. Happy to move it elsewhere.
3. Also moved CODEOWNERS since there's now .github.

[joelparkerhenderson]

Excellent, thank you!