Previously we supported one level of nested pseudo-element selectors
inside :host-context(), e.g. :host-context(:is(.foo, .bar)). This was
based on a regex-based approach. We could support deeper levels of
nesting by updating the regex, but using a regex approach prohibits us
from supporting arbitrary nesting.
Rather than just adding one more level to the existing expression, I've
added a new generator function which splits selectors on commas in a
parenthesis-aware way. This allows us to support arbitrary nesting.
It's likely we'll want to reuse this in other places where we're not as
careful today. We'll probably do this on a request-based basis, though.
Fixes#59176
(cherry picked from commit b478e91068)
04dd75ba94