Add support for bg-[paint(...)]

[devlzcode]

Expected behavior should be identical to bg-[url(...)] which resolves to background-image: url(...). Actual behavior is bg-[paint(...)] resolves to background-color: paint(...) which is not valid. Workaround is to either inline the style or [background-image:paint(...)].

[wongjn]

To note, another work-around is to type-cast explicitly as an image like bg-[image:paint(…)].