Improve the reftest analyzer code

[timvandermeij]

For the reftest analyzer I think the easiest solution would be to take over parts from the upstream one at https://github.com/mozilla/gecko-dev/blob/master/layout/tools/reftest/reftest-analyzer.xhtml since that one already got refactored to get rid of window usage. We shouldn't take over everything since there is also a large block of testing code in there, and we have made our own modifications to our version in https://github.com/mozilla/pdf.js/commits/master/test/resources/reftest-analyzer.js (both functional and code-wise, for example to not use XHTML and put the JS in a separate file), but it might save some time fixing this.

(Subtask of #13829)

[Snuffleupagus]

Also, if we're going to be updating/modernizing that code we should probably start with enabling ESLint/Stylelint/Prettier for that folder since otherwise it's just too easy to introduce (avoidable) bugs during the re-factoring.

[timvandermeij]

The security warnings are fixed by the PRs above. This issue is mainly about cherry-picking improvements from upstream and removing dead code.