[css-fonts] Add a ui-sans-serif keyword to go with ui-serif #4468
Description
A follow-up to #4107:
In that issue we agreed to add ui-serif, ui-monospaced, and ui-rounded to allow access to variants of system fonts other than the standard system-ui font. However, this set of names assumes that system-ui is a sans-serif style, which may not always be true.
There should be a ui-sans-serif keyword for authors who want to specifically request the system's sans serif style. The system-ui keyword would continue to be the primary font used by the system interface, regardless of style. These two keywords might map to the same font; the difference is in intent.