Render MW extension node wikitext with XML

MW extensions are XML not HTML, so we shouldn't build them as XML
to prevent HTML specific rules being applied, such as <source>
always being self closing.

Bug: 54577
Change-Id: I84af4a29cd1c4ae4d1db4f70a4012a8ad0f98bf6
Merge "Render MW extension node wikitext with XML"