Initial TemplateData commit

Registers a parser tag <templatedata> that should have a JSON
blob as content. The blob is then validated and normalised when
MediaWiki parses the page (e.g. during save and preview).

If there are validation errors, the save is aborted from the
extension hook and an error is displayed.

If all goes well, the normalised blob is stored in the database
(which can be retrieved through the API). And an HTML
representation of the template parameters is returned to the
wikitext parser to show where the <templatedata> was in the page.

The blob format is specified in spec.templatedata.json and
is validated in TemplateDataBlob::parse.

Bug: 44444
Change-Id: Icf305892a9512545a63f5a5280cc0d340c61585f