[css-text] text-shadow and inset

Due to persistent developer requests, we're planning on implementing
'inset' for text-shadow in Blink.  Does anyone have any strong
objections?  If not, can we update css-text (either level 3 or 4, I'm
not picky) to allow the keyword?

(It turns out that Skia has some features that make this not too hard
to implement in a performant manner, at least upon initial review.)

~TJ