Add new syntax: CSS3

[brenton-at-pieces]

Syntax:
CSS3

Guideline Criteria:
https://packagecontrol.io/packages/CSS3

[brenton-at-pieces]

It looks like "no_duplicate_extensions" is failing for "css".

Obviously, there is already a CSS syntax provided by Sublime. However, the commit being used for Sublime Packages repo is from 2019.

This new CSS repo is actively maintained.

I wonder about replacing the existing CSS syntax with this new CSS syntax.