In this patch we're introducing support for the SPAN element, which is the first
in the class of "any other tag" in the "in body" insertion mode.
This patch introduces the mechanisms required to handle that class of tags but
only introduces SPAN to keep the change focused. With the tests and mechanisms
in place it will be possible to follow-up and add another limited set of tags.
It's important that this not use the default catch-all in the switch handling
`step_in_body` because that would catch tags that have specific rules in previous
case statements that aren't yet added. For example, we don't want to treat the
`TABLE` element as "any other tag".
Props dmsnell.
Fixes#58907.
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