Update spec for relative imports in compileString()

[ntkme]

The current documentation for the url field in StringOptionsWithoutImporter is incorrect that it does not match the behavior defined in js-api-spec and actual implementation:

    it('url is used to resolve relative loads', () =>
      sandbox(dir => {
        dir.write({'foo/bar/_other.scss': 'a {b: c}'});

        expect(
          compileString('@use "other";', {
            url: dir.url('foo/bar/style.scss'),
          }).css
        ).toBe('a {\n  b: c;\n}');
      }));

• Fixes url documentation does not match js-api-spec dart-sass#1657.

[nex3]

This behavior doesn't match the behavior specified here. (Specifically, the part that says "If url is not null and importer is null, set importer to a function that always returns null.") However, after talking it over we've decided that the current actual behavior is likely to be more intuitive for users, so we should stick with it and update the documentation.

tl;dr: Please update the "Compiling a String" procedure to reflect this behavior as well.

[nex3]

I'm treating this as a fast-track proposal. Although the spec change itself is backwards-incompatible, the new specification wording matches the existing behavior of the reference implementation so this is not considered a backwards-incompatible change. I'll leave this open for two workdays from now in case anyone has any objections to this change.

[ntkme]

List of related PRs to make implementations consistent with the spec update:

• Fix and add specs for relative imports with compileString() sass-spec#1785
  • Update JS API spec.
• Use NoOpImporter for compileString unless url is a file: url dart-sass#1664
  • Fix dart-sass to set NoOpImporter when url is provided and is not a file: URL.
• Treat null importer as noOp if url is not a file: url dart-sass-embedded#83
  • Fix dart-sass-embedded to set NoOpImporter when url is not a file: URL.
• Set a FileSystemImporter if custom importer is not given for legacy API embedded-host-node#128
  • Fix embedded-host-node to force a FileSystemImporter for legacy API when filename is not provided.

@nex3 PTAL. Thanks!