Add scss as a supported language for syntax highlighting in documentation

[nshahan]

We commonly document the usage of visual customizations for angular UI components with .scss code. Sometimes it will do an OK job with auto-detection but if you add the scss hint to a triple tick code block it doesn't work well.

scss is a supported language in the highlightjs docs:
https://highlightjs.readthedocs.io/en/latest/css-classes-reference.html#language-names-and-aliases

[nshahan]

Probably needs coordination with dart-lang/pub-dev#1951

[jcollins-g]

This was addressed by @isoos in #1907.