Template folding is likely going to be a big 'hit' of a feature, but not
everyone will want it. Until CodeMirror prefs are introduced (T359498),
we need a way to control the rollout of template folding. This commit
adds $wgCodeMirrorTemplateFoldingNamespaces which acts similar to the
existing $wgCodeMirrorLineNumberingNamespaces.
We also move template folding to be part of CodeMirrorModeMediaWiki,
since the feature is unique to MediaWiki wikitext.
Move configuration settings to be part of the DataScript, thus removing
the need for the ResourceLoaderGetConfigVarsHook (which unnecessarily
makes the config settings available on every page load).
Other minor changes like adding missing JSDoc blocks.
Bug: T30684
Change-Id: I67518c0968f64c79e290f57b4884d30a161212d3
