Limit PDFPageProxy.getOperatorList to development mode and GENERIC builds

Given that this API method isn't used anywhere within the PDF.js library itself, except for the unit-tests, we can avoid including what's effectively dead code in e.g. the *built-in* Firefox PDF Viewer.

src/display/api.js

@@ -1552,6 +1552,9 @@ class PDFPageProxy {
     annotationMode = AnnotationMode.ENABLE,
     printAnnotationStorage = null,
   } = {}) {
+    if (typeof PDFJSDev !== "undefined" && !PDFJSDev.test("GENERIC")) {
+      throw new Error("Not implemented: getOperatorList");
+    }
     function operatorListChanged() {
       if (intentState.operatorList.lastChunk) {
         intentState.opListReadCapability.resolve(intentState.operatorList);