[GeckoView] Introduce a development mode constant to tell the viewers apart

Currently we have a couple of pre-processor checks, specifically for the GV-viewer, spread throughout the code. This works fine when *building* the viewer, however they're obviously ignored in development mode (i.e. `gulp server`).
This leads to a situation where the GV development viewer, i.e. http://localhost:8888/web/viewer-geckoview.html, behaves subtly different from its built version. This could easily lead to bugs, hence this patch introduces a development mode constant to hopefully improve things here.

Finally, in a follow-up to PR 15842, also ignores the `pageMode`-state since there's no sidebar available.

web/pdf_scripting_manager.js

@@ -269,7 +269,9 @@ class PDFScriptingManager {
           // NOTE: Always ignore the pageLayout in GeckoView since there's
           // no UI available to change Scroll/Spread modes for the user.
           if (
-            (typeof PDFJSDev !== "undefined" && PDFJSDev.test("GECKOVIEW")) ||
+            (typeof PDFJSDev === "undefined"
+              ? window.isGECKOVIEW
+              : PDFJSDev.test("GECKOVIEW")) ||
             isInPresentationMode
           ) {
             return;