[css-shadow-parts] Allow mixing implicit and explicit forwarding

[notwaldorf]

if it isn't already (it's not clear from the spec), it would be nice for part="ul => ul, * => archives-*" to be a valid syntax, i.e. "prefix everything except for ul"

[notwaldorf]

cc @tabatkins

[tabatkins]

Related: a way to forward everything but some blocklist, without having to explicitly forward some things. (That is, forward some things to nothing, and let everything else thru with a prefix.)