Move node-sass to npm script and drop Ruby Sass.

[bardiharborow]

This PR moves node-sass to a NPM script.

Per #20332, I've also dropped Ruby Sass support:

Compile Sass files with node-sass (no more Ruby options)

[hnrch02]

Do we know if libsass finally achieved feature parity with sass? A comparison in output for the unminified file would be handy, I think. (Unless this has already been examined and I'm OOTL.)

[bardiharborow]

Bootstrap already uses libsass by default, so this doesn't change that. The only differences in the output between libsass and Ruby Sass are to do with how whitespace is treated (I had to use --ignore-blank-lines to avoid diffing hundreds of added lines.) As for whether this will disrupt anyone, I posted a heads up on Slack and haven't heard any complaints.

[mdo]

Let's do it! Looks like tests are failing for some linting issues, otherwise looks good to me!

[bardiharborow]

@hnrch02 did you have any further concerns or shall we LGTM? I've fixed the linting issue.

[hnrch02]

Seems like I was just out of the loop on ruby Sass 😄 LGTM!