Handle meta-only transclusions as meta items

To achieve this we need to evaluate the DOM contents of
transclusion nodes to see if it consists solely of meta items
and whitespace.

To check for meta items we do a model registry match, but with an
additional parameter to exclude mwTransclusion types as a possible
result (as the first item may be a meta tag, but with a mw:Transclusion
typeof attribute).

Bug: 51322
Change-Id: I89a220350fb7e10e15f3682d21438539196a5846
Merge "Handle meta-only transclusions as meta items"