Implement sass --embedded in pure JS mode

[ntkme]

Closes #2325.

Implementation

The actual isolate dispatcher and compilation dispatcher are nearly unchanged. However, I had to replace isolate with worker communication, and mock tons of small things that do not work on node.

Testing

• All Dart embedded tests are passing. - GitHub CI has been updated to run these in this PR.
• All JS API tests are passing. - GitHub CI has been updated to run these in this PR.
• All Ruby API tests are passing.

[nex3]

Sorry that this has taken me so long to finish reviewing, there have been a lot of things to juggle this year.

I'm working on a major change (https://github.com/sass/dart-sass/tree/js-interop) to move Dart Sass to the new JS interop system. This includes creating a new package for defining typings and wrappers for the JS core library (as well as possibly pushing some changes upstream to dart:js_interop) as well as updating node_interop which will probably include dropping its compiler dependency. You can see the work in progress on each of these at https://github.com/sass/dart_js_core and https://github.com/pulyaevskiy/node-interop/tree/js_interop.

At this point it may make sense to wait on landing this PR until that work is finished. We're already closing in on that, so it shouldn't be too much more time.

[karolyi]

Rooting for this, as I can't use embedded sass on FreeBSD.

[verm]

Rooting for this, as I can't use embedded sass on FreeBSD.

Me too as I use it on FreeBSD and it works great.

[nex3]

The JS interop work is close to being done, at which point this will be unblocked.