[css-selectors] Standardise `:⁠(‑moz‑)first‑node` and `:⁠(‑moz‑)last‑node`

Right now, CSS Selectors 4 defines :first-child and :last-child, which works as such for the following:

span:last-child {
  background-color: lime;
}

<div>
  <span>This doesn’t match.</span>
  <span>This matches.</span>
</div>

<div>
  <span>This matches.</span>
  Some text after.
</div>

For :first-node and :last-node, this would work as follows:

span:last-node {
  background-color: lime;
}

<div>
  <span>This doesn’t match.</span>
  <span>This matches.</span>
  <!-- White-space and comments are ignored when
     - determining whether :last-node matches -->
</div>

<div>
  <span>This doesn’t match.</span>
  Some text after.
</div>

Mozilla Firefox already implements the previous in prefixed form and exposes it to the web.

Start with the CSS Selectors 4 child-index section and the linked MDN documentation for :first-child, :last-child, :first-node, and :last-node. Compare the issue’s HTML and CSS examples with Firefox’s prefixed behavior; done means the standard treatment of the selectors and their prefixed forms is clearly resolved.