[selectors] Let :is() have better error-recovery behavior than normal Selectors

This isn't captured in the current spec, but I remember earlier speculation that we could use :matches() as a way to get around the bad Selectors behavior of "a syntax error in one complex selector invalidates the whole sequence" that we're stuck with. Do we still think this is a worthwhile idea to pursue?

Spec-wise, what this would mean is defining the official syntax as :matches( <any-value> ), then split the result on top-level comma tokens, then attempt to parse each item as a <complex-selector>, and just ignore any invalid ones. (If all of them are invalid, the selector matches nothing.)

Then, if you're concerned about using a newer feature, you can just write your selectors like:

:matches( .foo || .bar, td.foo:first-child ) {
  /* styles for the bar cell */

An unfortunately, but relatively minor, tax for getting better error-recovery behavior. (If we do end up renaming it to :is(), it's even more minor.)

No repository file, test, or implementation entry point is named. Start by reviewing the current Selectors specification and this issue’s proposal; done means reaching and documenting an agreed behavior for per-item parsing and error recovery in :is().