add `:has-slotted`

[yisibl]

• Chrome: https://chromium-review.googlesource.com/c/chromium/src/+/5899636
• WebKit: https://github.com/WebKit/WebKit/pull/34599

[SebastianZ]

The change looks fine to me.

For what it's worth, Mozilla is also working on their implementation in https://bugzilla.mozilla.org/show_bug.cgi?id=1921747.

Sebastian