Bug 47579: Treat <source> as potential extension tag in the tokenizer

This fixes the deprecated <source> alias to <syntaxhighlight> for now.
Hopefully we can remove this exception eventually once bots have cleaned up
the remaining <source> usage.

Change-Id: Ifc8776419755b669d712b5bb028633cdd60e8c41
Merge "Bug 47579: Treat <source> as potential extension tag in the tokenizer"