Discrepancy between the default value of importer in Node API and Dart API

[ntkme]

In Node API, importer defaults to NoOpImporter() or null (which becomes FilesystemImporter('.') later in Dart API):

dart-sass/lib/src/node/compile.dart

Lines 77 to 78 in 33f18c4

	importer: options?.importer.andThen(_parseImporter) ??
	(options?.url == null ? NoOpImporter() : null),

In Dart API, importer defaults to FilesystemImporter('.'):

dart-sass/lib/src/async_compile.dart

Line 111 in ba38c09

importer ?? FilesystemImporter('.'),

This behavior difference has caused bugs like #1657 and sass/dart-sass-embedded#82.

While it is not too hard to fix the those bugs, it is probably more important to address this discrepancy and be clear on what is the correct behavior.

[ntkme]

https://github.com/sass/sass-spec/blob/126d17ff9f77372b6f68cb0d43dd357165519c93/js-api-spec/compile.test.ts#L124-L133

Assuming the JS API spec referenced above is correct and desired behavior, the current implementation is actually correct for most part.

• url documentation does not match js-api-spec #1657 is a documentation issue.
• Imports shall not be resolved relative to working directory of dart-sass-embedded process  dart-sass-embedded#82 is a bug in the host.