Parse less and scss files

This pull request is for [Simple support for LESS/SCSS](https://trello.com/c/KoK6kPtp/1190-m-simple-support-for-less-scss-in-key-brackets-features).


Shouldn't this be `_nextTokenSkippingComments()`, like the similar function below this?  It's an edge case, but there could be a comment inside a property (anywhere whitespace could be inserted), and the comment could contain a ";"...

More broadly -- how many places are there where we want to skip whitespace but not comments?  Probably not many, and comments are allowed pretty much anywhere whitespace is allowed.  Maybe we should move most callers over to `_nextTokenSkippingComments()`?