fix(issue:4268) nested pseudo-selector parsing

[puckowski]

What:

Fix for issue #4268.

Why:

Issue #4268 concerns valid CSS, see example CodePen: https://codepen.io/puckowski/pen/azooPzZ?editors=1111

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

Lines of code delta: 38 added (including tests)

The following Less

a:is(.b, :is(.c)) {
  color: blue;
}

a:is(.b, :is(.c), :has(div)) {
  color: red;
}

becomes:

a:is(.b, :is(.c)) {
  color: blue;
}
a:is(.b, :is(.c), :has(div)) {
  color: red;
}

[puckowski]

I submitted a few PRs. I expect rebase(s) will be needed in order to merge all of them, so let me know when a rebase is needed.