Collect telemetry on all unsupported features

[emalysz]

In #11815, we collected telemetry on what error triggered the fallback bar. The first error message is what is shown in the fallback bar. We need expand this and collect telemetry on all error messages to give us a better understanding of the frequency of each type of error.

Example with more than one error:
https://www.irs.gov/pub/irs-pdf/f1040.pdf

[Snuffleupagus]

One possible way of doing this could be to decouple the showing of the fallback bar from the reporting of telemetry data, to not complicate e.g. the PDFViewerApplication.fallback method unnecessarily; perhaps something along these lines:

• Remove the featureId bit from https://searchfox.org/mozilla-central/rev/559b25eb41c1cbffcb90a34e008b8288312fcd25/browser/extensions/pdfjs/content/PdfStreamConverter.jsm#499
• Split off https://searchfox.org/mozilla-central/rev/559b25eb41c1cbffcb90a34e008b8288312fcd25/browser/extensions/pdfjs/content/PdfJsTelemetry.jsm#29-32 into a new (suitably named) function.
• Add a new case, e.g. "unsupportedFeature" to the switch-block in ChromeActions.reportTelemetry (similar to e.g. the "documentStats" case) which calls the new PdfJsTelemetry function.
• Always report telemetry in PDFViewerApplication.fallback, by adding something like

  this.externalServices.reportTelemetry({
    type: "unsupportedFeature",
    featureId,
  });

  immediately after line

  pdf.js/web/app.js

  Lines 865 to 869 in 8fc1126

  	fallback(featureId) {
  	if (
  	typeof PDFJSDev === "undefined" ||
  	PDFJSDev.test("MOZCENTRAL || GENERIC")
  	) {

[emalysz]

Upstream bug to add telemetry in Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1643402