message: change pdf.js to PDF.js
diff: ### Overview
  -The pdf viewer is based on [pdf.js](https://mozilla.github.io/pdf.js/) by Mozilla Foundation. Files are from the [prebuilt download version](https://mozilla.github.io/pdf.js/getting_started/#download). The reason we do not use `web/pdf_viewer.js` from `pdfjs-dist` package is that `pdf_viewer.js` does not provide features we need. `pdf_viewer.js` is much simpler than `viewer.js`. See [link](https://github.com/mozilla/pdf.js/issues/9318).
  +The pdf viewer is based on [PDF.js](https://mozilla.github.io/pdf.js/) by Mozilla Foundation. Files are from the [prebuilt download version](https://mozilla.github.io/pdf.js/getting_started/#download). The reason we do not use `web/pdf_viewer.js` from `pdfjs-dist` package is that `pdf_viewer.js` does not provide features we need. `pdf_viewer.js` is much simpler than `viewer.js`. See [link](https://github.com/mozilla/pdf.js/issues/9318).
  Mozilla [asks](https://mozilla.github.io/pdf.js/getting_started/) web developers to reskin `viewer.html` because Firefox users would think bugs of the viewer on the web site are ones of Firefox and would report them to the pdf.js team. See [link](https://github.com/mozilla/pdf.js/issues/5609). Our usage does not cause such a problem.
  -We provide additional features by setting up new event listeners in `latexworkshop.js` for DOM objects in `viewer.html`. We do not and should not override functions defined by pdf.js.
  +We provide additional features by setting up new event listeners in `latexworkshop.js` for DOM objects in `viewer.html`. We do not and should not override functions defined by PDF.js.
  We can see the [changes](https://github.com/James-Yu/LaTeX-Workshop/compare/a0b97a9...a5f8c04#diff-ff661e0ff756ae1ff026c0e8f4561d0e) we have made to `viewer.js`. We had better find a way to achieve this without modifying `viewer.js`.
  