[selectors-4] Add CanIUse panels #1193 (#14120)

Enables Include Can I Use Panels metadata and adds caniuse=<feature>
attributes on 14 headings and dfns to render browser-support panels
next to each pseudo-class and selector documented in caniuse.com's
database.

Anchors covered: attribute-case, descendant-combinators, matches,
negation, relational, the-dir-pseudo, the-any-link-pseudo,
the-focus-within-pseudo, placeholder, the-default-pseudo,
indeterminate-pseudo, range-pseudos, optional-pseudo,
the-nth-child-pseudo.

Not covered: #first-line from the original 2017 issue. The
::first-line pseudo-element is now defined in css-pseudo-4, so its
CanIUse panel belongs there.