Move linting code to under Stanc, tests

[WardBrian]

Closes #105

[jsoules]

I'm fine with this as is, but spent way too much time making fussy little recommendations.

Thanks for including the tests; they're great for clarifying things. Looking at the coverage reports, you may want to include a test for both warnings and errors from included files.

You may also wish to write a test for stancErrorsToCodeMarkers, it's pretty trivial but I noticed it was currently uncovered, and it would probably cover stancMessageToMarker by incorporation. I suppose the behaviors to confirm would be that there are no empty elements in the list, that the errors are all mapped before the warnings, and that the messages track with the severity or something?

[WardBrian]

I added the tests you requested, expect for included files -- since we don't currently support that, I just removed the code. A bunch of stuff would need to change first anyway ;)