Test that all languages have grammars

This will make CI fail if someone adds a new language but neglects to add a new grammar for it. This should make it easier for people to review PRs, as CI will help them to make sure a new grammar gets added.

However, we currently support some languages that have no grammars, and we may support more in the future. So you can explicitly mark the language as having no grammar by setting `tm_scope: NONE` in languages.yml.

@vmg @bkeepers @arfon Do you think this is a good idea?