[api-minor] Remove Outliner from the official API

As far as I can tell `Outliner` is only exposed in the API because we need to access it when running some of the reference-tests, but is otherwise not used.
Hence this seems like something that should be kept *internal* and thus only exposed in TESTING-builds.

src/pdf.js

@@ -80,6 +80,12 @@ const pdfjsVersion =
 const pdfjsBuild =
   typeof PDFJSDev !== "undefined" ? PDFJSDev.eval("BUNDLE_BUILD") : void 0;
 
+if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("TESTING")) {
+  globalThis.pdfjsTestingUtils = {
+    Outliner,
+  };
+}
+
 export {
   AbortException,
   AnnotationEditorLayer,
@@ -109,7 +115,6 @@ export {
   noContextMenu,
   normalizeUnicode,
   OPS,
-  Outliner,
   PasswordResponses,
   PDFDataRangeTransport,
   PDFDateString,