The parsed content of JSON files in the gadget is made available from the
gadget's JS files via require(). That is, MediaWiki:Gadget-data.json (or
Gadget:data.json) is available as `require('./data.json')`. This is
supported for both MediaWikiGadgetsDefinitionRepo and
GadgetDefinitionNamespaceRepo. The JSON parsing is done server-side.
JSON can only be used in "package" gadgets - in which the JS files can
also be invoked via require().
Also added a test for GadgetResourceLoaderModule.
Bug: T198758
Depends-On: Ib4556d09c4d393269e32771aab00f59a5f630e1b
Depends-On: Id4589a597ccfc4266b3e63d10f75b146aa7a287a
Change-Id: I21acb46cdd244a39b5cc6963aa763f0113bd1e38
