Support sass-embedded (dart-sass-embedded)

[ntkme]

Disclaimer: This is a draft PR just for interest check. It is not how sass-embedded gem is indented to be integrated, as I only hacked quickly to get all tests to pass as a proof of concept.

• Sprockets 4.0: alternative CSS processors? #661

With libsass and sassc-ruby being deprecated and no replacement for a long time, I see the community started to think about other approach like rails/dartsass-rails, where many sprockets features today would be missing (e.g. ability to write custom sass functions & importers in ruby). However, the situation has changed, I'm happy to announce the 1.0 release of sass-embedded gem that provides a ruby language API for dart-sass-embedded compiler.

First of all, sass-embedded uses the officially support dart-sass-embedded compiler and is very different from the previous two ruby sass implementations. The sass-embedded ruby API closely mirrors the current dart-sass JS API in order to properly support with all the latest features.

Hers are the major differences:

1. sass-embedded gem uses Sass::Embedded namespace that creates a conflict with the old sass gem, meaning it is a breaking change. I do not see it as a big problem as it's been years that original sass gem was deprecated by sassc gem.
2. The previous default compact output style is no longer supported, the new default is expanded.
3. Custom functions work differently. There is no module Sass::Script::Functions based custom function loading. The API now takes a Hash of sass function signatures and methods. This new design is due to the addition of ArgumentList type in dart-sass which cannot be mapped to ruby arguments without ambiguity, as well as the need of access tracking of keyword arguments. In this proof of concept, I did a hack that converts the existing module to a Hash so that I can get tests to pass, which is not how it is supposed to work.
4. Custom importers work differently. There is no module Sass::Importers, importers are now just a list of objects.
5. The ruby Sass::Value types are different from old Sass::Script::Value types.
6. sass-embedded requires ruby >=2.6. The CI tests will fail.

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I opened this mainly to seek comments from the maintainers and the community. I'd like to know if the community is interested, and if so whether the rails team would like to driver the real development of this integration.

[ntkme]

Closing in favor of https://github.com/ntkme/sassc-embedded-shim-ruby