Feature/gherkin linting

[Lewiscowles1986]

Proposal

Fixes #82

• Uses GitHub action (can be merged into single file. Kept separate so potential merge conflicts can be avoided
• Uses NPM / JS (can pivot and search for something else if something suitable exists)
• Adds config file (which may be in need of iteration, it was borrowed from Moodle, but had some spelling mistakes I fixed to avoid errors in a likely outdated file)

Let me know if this is unsuitable. If you really want to have fun and test this:

1. clone the branch
2. npm install
3. npm run lint && echo success || echo failure (should contain "success" and not contain "failure")
4. change the indentation of a .feature file (I tested by removing all indentation)
5. npm run lint && echo success || echo failure (should contain "failure")

[schlessera]

Thanks for the PR, @Lewiscowles1986 !