We have a number of base-classes that are only intended to be extended, but never to be used directly. To help enforce this during development these base-class constructors will check for direct usage, however that code is obviously not needed in the actual builds.
*Note:* This patch reduces the size of the `gulp mozcentral` output by `~2.7` kilo-bytes, which isn't a lot but still cannot hurt.
To avoid having to request and await various browser data early during the PDF Viewer initialization, we can include that data in the existing preference fetching instead. With other planned changes to the PDF Viewer, the current situation would only become worse over time.
*Note:* Technically this data aren't preference-values, however we're already including other non-prefs in this list (e.g. `isInAutomation`) and doing it this way simplifies the overall implementation.
This old method is essentially just adding, a small amount of, unnecessary indirection and we can easily invoke `PDFViewerApplication.open` directly from the extension-specific code instead.
The `DefaultExternalServices` code, which is used to provide build-specific functionality, is very old. This results in a pattern where we first initialize `PDFViewerApplication.externalServices` and then *override* it for the different builds.
By converting `DefaultExternalServices` into a "regular" class, and leveraging import maps, we can directly initialize the correct instance depending on the build.
