Add CodeMirror support for the infobox tag and its children in extension.json

[the-dragons-studio]

At the moment, PortableInfobox does not define how to syntax highlight the infobox tag, leading to everything within, including wikitext, being harder to edit. PortableInfobox should probably define some tags to be syntax highlighted in extension.json's CodeMirrorTagModes; most criticially, those that contain wikitext like the format, default and label tags.

Some ideas:

• Use text/mediawiki for the label, format and default tags.
• If it doesn't interfere with subtags, mark infobox as text/xml or similar