Upgrade `systemjs` to the most recent version

Currently we use the systemjs 0.x version, but the most recent version as of writing is the 4.x version. Updating is a bit difficult because of API changes, but should in return simplify the logic and also allow us to e.g., retry #10953 afterwards since that it blocked on this update.

More information can be found on https://guybedford.com/systemjs-2.0.

[Snuffleupagus]

For anyone working on this, please make sure to carefully test your patch prior to submitting a PR. This entails, at the very least:

• Ensure that the development viewer works correctly, by running gulp server and:
  • Open the development viewer, with http://localhost:8888/web/viewer.html, in a browser and make sure that the viewer both loads and works correctly without any Errors in the console.
  • Run the unit-tests directly in a browser, with http://localhost:8888/test/unit/unit_test.html, and ensure that all the same tests pass with the patch as with the master branch.
• Run gulp unittest locally, and ensure that all tests pass.
• Run gulp fonttest locally, and ensure that all tests pass.
• Run gulp browsertest locally, and ensure that all tests pass.
• Finally run gulp test locally, and ensure that all tests pass.

Useful resources:

• https://github.com/mozilla/pdf.js#getting-the-code
• https://github.com/mozilla/pdf.js/wiki/Contributing

[timvandermeij]

Some work is currently in progress to reduce the dependency on/usage of SystemJS, so it might be easier to wait until that’s done so the remaining upgrade work will be easier.