WIP support for LaTeX citations in text editor

[krassowski]

Closes #7.

TODO:

• BLOCKER: cannot add multiple citations for the same item; this is because citeproc requires unique IDs; need to either (a) change the format to citationID:source|itemID,source|itemID - which gets long and unwieldy for LaTeX, or (b) do it transparently in the background? Could do (a) easily, but possibly for forward-compatibility it would be better to implement (b) immediately (unless it turns out prohibitive);
  • ideally we could have source + itemID aliases to allow \cite{surnameYear}, but not a blocker
• BLOCKER: remove reliance on defaultFormat in index.ts as it adds <i> to latex documents breaking bibliography; instead push it down to HTML formatter (and remove the defaultFormat in general)?
• move refocus() logic to adapter (and possibly more?)
• better mime type detection (app.docRegistry.getFileTypeForModel(contentsModel))
• add styles support