Fix regressions in source maps

[wkillerud]

This PR fixes source maps that broke in 6.0.0 in some cases, by transforming the sourcemaps generated by the new Sass JS API to match the old output.

• Relative file paths instead of absolute file:// URLs.
• Replace data:<file contents> with relative path to file.
• Update tests to only accept the relative path format, regardless of compiler.

Fixes #875

[wkillerud]

@xzyfer, when you have the time, I have a regression fix for sourcemaps in gulp-sass.

The CI tests were a bit flaky on Windows around the removal of the result directory, so I took the liberty of upgrading to a version of rimraf that is more stable (according to the changelog). Its minimum version is Node 14, so I dropped 12 from the test matrix.

[xzyfer]

Thanks for all your work @wkillerud. This all looks reasonable to me.