move CSS build to webpack (#9983)

- added new 'make webpack' target
- deprecated 'make js' and 'make css'
- extend webpack config to load the less files
- updated docs

I had to rename the source file of `arc-green.less` to avoid generating
a useless JS entrypoint via webpack-fix-style-only-entries which would
not work with different source/destination filenames. I hear that there
should be cleaner solutions possible once we upgrade to Webpack 5.

Co-authored-by: zeripath <art27@cantab.net>

.editorconfig

@@ -7,6 +7,9 @@ insert_final_newline = true
 trim_trailing_whitespace = true
 end_of_line = lf
 
+[*.md]
+trim_trailing_whitespace = false
+
 [*.go]
 indent_style = tab
 indent_size = 8