Add github action to shellcheck bash repo

[glennj]

This is to ensure code quality.

Inspired by shellcheck usage in factor-test-runner.

Todo:

• Fix any findings in bats files
• Fix any findings in example solutions
• Add shellcheck.yaml to .github/workflows

[glennj]

I'll exclude

• check SC1091 ("not following 'source somefile'")
• single file exercises/practice/markdown/markdown.sh, the purposefully bad file to refactor