Feature-test moz-chunked-arraybuffer before use

Resubmission of https://github.com/mozilla/pdf.js/pull/5457, rebased to master. The previous one was closed with the following comment:

> Closing for now. #5448 has landed, so the issue itself should be fixed with less code than in this PR. Optimizions for the landed fix can be proposed in a new (rebased) PR.

This PR indeed has a bit more code, but that is caused by the fact that this PR uses feature detection instead of try-catch.
The other diff is just some re-indention after moving the Mozilla-specific APIs between preprocessor conditionals.