This CL factors out the decision-making logic whether a property key should
be treated as a "property" or "element" into LookupIterator::Key, which can
be constructed on its own, allowing use sites to take this distinction into
account before constructing a LookupIterator from the Key, without needing
to duplicate the logic.
This also makes the assortment of LookupIterator constructors more uniform.

Bug: chromium:1031175
Change-Id: I81d7b11ab7e4915f5c05668138e6e0c51ae11821
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1962272
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65672}