resources: Address Dart Sass deprecation of global built-in functions

[jmooring]

See https://github.com/sass/dart-sass/releases/tag/1.80.0

Fixes #12961

[jmooring]

#12961 (comment)

I guess adding some kind of identifier as the first arg to CreateVarsStyleSheet would solve it.

To avoid hardcoded identifiers, I moved the constant declarations from the tpl/css package to the sass package, which was previously an "internal" package. I had to make it "not internal" in order to export the constants.