[css] Enable drop image into css to insert url

Follow up on #142990 ## Feature request I should be able to drag an image file from the explorer into a css file to insert a url. For example, with the workspace structure: ``` images/ cat.gif styles/ main.css ``` If I drag `cat.gif` from the VS Code explorer into `main.css`, I should get a url such as: ```css body { background-image: url(/images/cat.gif); } ``` Users — especially beginners — may find drag and drop more natural than writing out the image path