feature/issue 1199 url references in CSS bundling

[thescientist13]

Related Issue

resolves #1199

Summary of Changes

1. Support bundling url references in CSS files with hashed filenames
2. Add test cases
3. Update Styles and Assets docs

TODO

1. inline <style> scratch dir assets handling and test case
2. basePath support
3. bundle / hash filenames
4. docs
5. clean up console logs / refactoring

Questions

1. Is our resolution algorithm too naive to map back url references to assuming the root of the user's workspace when dealing with relative paths, e.g. ../? - I think it's fine for now, if it breaks later on a particular use case, we can improve it then with a clear reproduction
2. to the above point, what about if using CSS Modules, e.g. with { type: 'css' } - tracking in apply CSS optimizations and bundling to CSS (in) modules #1227
   • as per import attributes for standard module formats; CSS and JSON  #923 , which would get transformed into a JS object response presently..., does that mean everything would have to go into assets/?
3. rename assets/ to public/ for general asset copying (could lead to duplicate copies) - align assets and build output directory naming conventions #1219
   • would mean changing default greenwood config output build directory name