SASS Modules (@forward and @use)

[wongjn]

Dart Sass 1.23.0 dropped recently and with it, a new module system.
It would be great if this was added into the language features for SCSS/SASS.

Documentation pages for each of the new at-rules:

• @use
• @forward

[wongjn]

I've made an extension that adds highlighting in SCSS until this gets added to core.

[octref]

Pending merge on atom/language-sass#269. I'll watch that PR. Thanks @wongjn for your work 👍