Re-factor how the GenericL10n class fetches localization-data

- Re-factor the existing `fetchData` helper function such that it can fetch more types of data, and it now supports "arraybuffer", "json", and "text".
   This only needed minor adjustments in the `DOMCMapReaderFactory` and `DOMStandardFontDataFactory` classes.[1]

 - Expose the `fetchData` helper function in the API, such that the viewer is able to access it.

 - Use the `fetchData` helper function in the `GenericL10n` class, since this should allow fetching of localization-data even if the default viewer is run in an environment without support for the Fetch API.

---
[1] While testing this I also noticed a minor inconsistency when handling standard font-data on the worker-thread.

test/unit/pdf_spec.js

@@ -44,6 +44,7 @@ import {
 } from "../../src/display/api.js";
 import {
   DOMSVGFactory,
+  fetchData,
   getFilenameFromUrl,
   getPdfFilenameFromUrl,
   getXfaPageViewport,
@@ -78,6 +79,7 @@ const expectedAPI = Object.freeze({
   createValidAbsoluteUrl,
   DOMSVGFactory,
   FeatureTest,
+  fetchData,
   getDocument,
   getFilenameFromUrl,
   getPdfFilenameFromUrl,