gdlint: Check for docstrings

[rgson]

Could gdlint warn about missing docstrings at the top of scripts?

I'm thinking of something similar to pylint's "Missing module docstring (missing-docstring)".

The official style guide recommends adding docstring comment near the top, after tool, class_name and extends.

[Scony]

That's too pedantic to add as a regular check, but I agree it will be a pretty cool remark-check. For that, however, we will need check importance levels just like in pylint. So it will be implemented for 4.x.