getElementsByTagName(), inexplicably, takes quadratic time (relative
to the number of found elements).

Use DOMCompat::querySelectorAll() instead (provided by Parsoid).
Also replace some uses with DOMCompat::querySelector() for clarity.

Add a hack to allow Phan to pass (adapted from DiscussionTools),
because Parsoid has some weird class aliases to allow swapping DOM
implementations.

Set maxImages and maxHeadings to very high values (we can remove it
later). Even 100,000 items should be processed in just a few seconds.
You'll probably run into other parser issues before you reach that
point.

Bug: T317070
Change-Id: I3741291edd28dff5e98074b1cf18dc8907681586