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).


You're absolutely right to use `nextTokenSkippingComments()` in this case. I checked all other calls of `_nextTokenSkippingWhitespace()` and they're used correctly so that we don't miss any user comment appearing before a nested rule.