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.

[ntkme]

@nex3 Any feedbacks on my last two comments above?

[ntkme]

@nex3 Pinging again regarding how to deal with legacy js package vs new js_introp:

• The legacy js package has been officially marked discontinued.
• Both the legacy js package and new js_introp provides @JS annotations. This issue is addressed in 0.7.0 of legacy js:

  Breaking Change: Moved annotations to single location in dart:_js_annotations. This makes it easier to avoid collisions with dart:js_interop's @JS, as you will now see a static error instead of it resolving in favor of the package definition. However, since this may result in breakages in previously working code, this is treated as a breaking change.

• Ideally node_interop should be rewritten with new js_interop but nothing is happening. On the other hand, node_interop cannot be easily upgraded to js 0.7.x due to js 0.7.x requires dart >=3.1.x, while node_interop's dev dependencies build_node_compilers do not support dart 3.x at all. This means node_interop is effectively stuck at js 0.6.x as of now, which means we have @JS conflict that we cannot mix old js and new js_interop code in the same file.

So honestly I think the way js interop is handled in this PR is already at best effort. - That new code only uses js_interop and it's currently isolated from the legacy js code.

Any more thoughts? Would you mind take another look at this?

[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.