Enable typechecking of most JS files in src/annotator except for:

 - The entrypoint index.js file. This cannot be usefully checked since
   it imports many CoffeeScript files
 - pdf-sidebar.js - This extends a class currently written in
   CoffeeScript
 - src/annotator/anchoring/pdf.js - This requires types for various
   PDF.js globals to be defined
 - The `annotator/plugin` directory