[css-backgrounds] Change canonical serialization for box-shadow #2305

The spec declares the syntax for a shadow in box-shadow to be:

<shadow> = inset? && <length>{2,4} && <color>?

However, Firefox, WebKit, and Chrome all serialize this as <color>? && <length>{2,4} && inset?. This matches the serialization for drop-shadow() (see https://drafts.fxtf.org/filter-effects/#funcdef-filter-drop-shadow) and text-shadow (see https://drafts.csswg.org/css-text-decor-3/#text-shadow-property), which both are declared as <color>? && <length>{2,3}.

For consistency, <shadow> should be defined as:

<shadow> = <color>? && <length>{2,4} && inset?

in order for serialization to match 3 out of the 4 major browsers and text-shadow/drop-shadow().