Add declarative DOM Parts syntax

This CL adds the ability to construct DOM Parts during document
parsing via "fake" processing instructions:

<section>
  <h1 id="name">
    <?child-node-part?> <?/child-node-part?>
  </h1>
  Email: <?node-part metadata?><a id="link"></a>
</section>

In this example, <?child-node-part?><?/child-node-part?> will
be left in the document, but a ChildNodePart will also be constructed
with previous/next endpoints of those two comment nodes. And the
<?node-part> will be left in place, but a NodePart will be attached
to the <a> element.

Note that <?foo?> is exactly equivalent to <!--?foo?-->, and both
are tested in the new WPT.

This CL also:
 - adds a virtual test suite that makes sure no functionality breaks
   or leaks when DOMPartAPI is disabled.
 - revamps the WPT, adding a `assertEqualParts()` that checks a given
   parts list via the type of parts and the metadata. This is then
   used everywhere.
 - add testing of declarative DOM Parts for three cases:
   1. main document parsing (passes with this CL)
   2. template parsing (doesn't yet pass)
   3. cloned templates (not even sure if this should ever pass)

Bug: 1453291,1465062
Change-Id: I2eaef72a21fc2deaa251deb0731b5ff55ef19d8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4686189
Reviewed-by: David Baron <dbaron@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1170849}