Run cli compilations in parallel dart isolates

[ntkme]

This PR enables parallel complications in isolates when running the pure dart version of sass cli.

$ sass --version
1.66.1
$ time sass bootstrap/scss:css
sass bootstrap/scss:css  0.39s user 0.06s system 99% cpu 0.450 total

$ sass --version
1.66.2-dev
$ time sass bootstrap/scss:css
sass bootstrap/scss:css  0.44s user 0.08s system 145% cpu 0.353 total

The goal of this PR is to improve the performance of https://github.com/rails/dartsass-rails, which invokes the cli directly.

[nex3]

@ntkme The test failure here suggests that we may need to print all messages in the main isolate to ensure they don't end up stepping on one another.

[ntkme]

@nex3 I think the issue is that the internal implementation of print uses writeln, which actually write the line and newline character in two separate calls. I think by adding the newline character to the buffer and then use write instead of writeln, it should be able to avoid this issue.

[ntkme]

@nex3 I put up small hack that overrides print for dart vm. Obviously it's not the most elegant solution but supposedly should work. I think a cleaner solution would be to add newline directly to all StringBuffers, and then replace all print with stdout.write.

[nex3]

Would you mind filing an issue against the Dart SDK about print() not being thread-safe? It seems like the best way to fix that is probably in the SDK itself.

[ntkme]

Would you mind filing an issue against the Dart SDK about print() not being thread-safe? It seems like the best way to fix that is probably in the SDK itself.

dart-lang/sdk#53471