util.join() doesn't handle absolute path on Windows correctly

We use the Webpack pipeline on Windows with sass-loader, postcss-loader, and css-loader. Then we found the generated source paths went crazy: F:\Projects\web-project-boilerplate\src\scss\index.scss just becomes F:/Projects/web-project-boilerplate/src/scss/F:/Projects/web-project-boilerplate/src/scss/F:/Projects/web-project-boilerplate/src/scss/index.scss.

As I followed the codebase in the pipeline, I noticed that the sass-loader use relative path, then the postcss-loader and css-loader turn to use absolute path for "sources" field in the sourceMap. In addition, the absolute path on Windows starts with a symbol of the disk driver, e.g. F: in our case. When it comes to sourceMap's util.join(), it only detects absolute path starts with /

https://github.com/mozilla/source-map/blob/c3a8f5a2a4cc7ac5bc6c06bc93745d3c57bd8860/lib/util.js#L228

So it joins multiple paths across the loaders.

Also, the util.isAbsolute doesn't deal with the Windows absolute path as well.

https://github.com/mozilla/source-map/blob/c3a8f5a2a4cc7ac5bc6c06bc93745d3c57bd8860/lib/util.js#L241

Appendix:
Here is a case of how expressjs test the absolute path:

exports.isAbsolute = function(path){
  if ('/' === path[0]) return true;
  if (':' === path[1] && ('\\' === path[2] || '/' === path[2])) return true; // Windows device path
  if ('\\\\' === path.substring(0, 2)) return true; // Microsoft Azure absolute path
};

I'm not sure we need to handle the azure absolute path, but I would be better if we had window device path included.

Thanks

Start in lib/util.js at util.join() and util.isAbsolute(), using the reported Windows drive-letter path as the reproducer. Check how both entry points treat paths such as F:\Projects\... alongside slash-prefixed paths. Done means absolute Windows paths are recognized and are not repeatedly joined across the source-map loader pipeline; run the existing project tests afterward.