Feature request: Basic linting for composite actions

[JavaScriptBach]

What do you think about adding some basic support for linting composite action.yml files? For example:

• Missing shell: attribute for steps
• Attempts to access ${{ secrets.* }}

These are common mistakes that I make when writing actions. In general this project has helped me a lot when writing workflows, and I'd love to get its benefits extended to writing actions!

[AndreyNautilus]

Seems like a duplicate of #46

[rhysd]

Yes, this is duplicate of #46.