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.
Change NONE to none

NONE is a little shouty.
Add some docs for tm_scope
Merge pull request #1756 from github/test-for-grammars

Test that all languages have grammars