For #11961: collect telemetry on all unique unsupported features that trigger fallback error.

This expands upon the telemetry we are collecting that shows the fallback error.

Author: emalysz

web/app.js

@@ -869,6 +869,20 @@ const PDFViewerApplication = {
       typeof PDFJSDev === "undefined" ||
       PDFJSDev.test("MOZCENTRAL || GENERIC")
     ) {
+      if (featureId) {
+        this.externalServices.reportTelemetry({
+          type: "unsupportedFeature",
+          featureId,
+        });
+      } else {
+        for (const unsupportedFeature of this._delayedFallbackFeatureIds) {
+          this.externalServices.reportTelemetry({
+            type: "unsupportedFeature",
+            featureId: unsupportedFeature,
+          });
+        }
+      }
+
       // For PDFs that contain script and form errors, we should only trigger
       // the fallback once the user has interacted with the page.
       if (this._delayedFallbackFeatureIds.length >= 1 && this._hasInteracted) {