Replace dependency on dartsass-ruby with sass-embedded

[johnnyshields]

As per this thread, the dartsass-ruby gem is a compatibility shim which provides legacy SassC features on Dart Sass.

When Dart Sass 2.0 is released, many of these features will become impossible to support.

Therefore, we should get rid of the dartsass-ruby dependency and replace it with a direct integration to sass-embedded. This involves generally replacing any references to the ::SassC namespace. The sass-embedded documentation is here.

To recap:

• Current dependency pattern: dartsass-sprockets --> dartsass-ruby --> sass-embedded

• Future dependency pattern: dartsass-sprockets --> sass-embedded

A PR would be welcome for this.

[johnnyshields]

This is done