layout: Simplify and generalize the usage of pseudo-elements (#36202)

- Remove the last remaining Servo-specific PseudoElement enum from
  layout. This was made to select `::before` and `::after` (both eager
  pseudo-elements), but now `traverse_pseudo_element` is called
  `traverse_eager_pseudo_element` and should work on any eager pseudo
  element.
- Expose a single way of getting psuedo-element variants of
  ThreadSafeLayoutElement in the Layout DOM, which returns `None` when
  the pseudo-element doesn't apply (not defined for eager
  pseudo-elements or when trying to get `<details>` related
  pseudo-elements on elements that they don't apply to).
- Ensure that NodeAndStyleInfo always refers to a node. This is done by
  making sure that anonymous boxes are all associated with their
  originating node.

These changes are prepatory work for implementation of the `::marker`
pseudo-element as well as ensuring that all anonymous boxes can be
cached into the box tree eventually.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/meta/css/cssom-view/elementFromPoint-list-001.html.ini

@@ -1,15 +0,0 @@
-[elementFromPoint-list-001.html]
-  [<li>Image Outside 2</li>]
-    expected: FAIL
-
-  [<li>Outside 2</li>]
-    expected: FAIL
-
-  [<li>Image Outside 1</li>]
-    expected: FAIL
-
-  [<li>Outside 3</li>]
-    expected: FAIL
-
-  [<li>Outside 1</li>]
-    expected: FAIL

tests/wpt/meta/intersection-observer/intersection-ratio-with-fractional-bounds-2.html.ini

@@ -1,3 +0,0 @@
-[intersection-ratio-with-fractional-bounds-2.html]
-  [IntersectionObserver ratio with fractional bounds]
-    expected: FAIL