Add `src/types/pdfjs.js` module which defines the subset of the PDF.js
viewer API that the client actually uses and make use of it in
`src/annotator/anchoring/pdf.js` and `src/annotator/plugin/pdf-metadata.js`
to typecheck these files.

As well as helping to catch errors and JSDoc mistakes in these files,
this should also make future PDF.js upgrades easier because we can see
what PDF.js APIs the client actually relies on.