In Webpack version `5.99.0` the way that `export` statements are handled was changed slightly, with much less boilerplate code being generated, which unfortunately breaks our `tweakWebpackOutput` function that's used to expose the exported properties globally and that e.g. the viewer depends upon.
Given that we were depending on formatting that should most likely be viewed as nothing more than an internal implementation detail in Webpack, we instead work-around this by manually defining the structures that were previously generated.
Obviously this will lead to a tiny bit more manual work in the future, however we don't change the API-surface often enough that it should be a big issue *and* the relevant unit-tests are updated such that it shouldn't be possible to break this.
*NOTE:* In the future we might want to consider no longer using global properties like this, and instead rely only on proper `export`s throughout the code-base.
However changing this would likely be non-trivial (given edge-cases), and it'd be an `api-major` change, so let's just do the minimal amount of work to unblock Webpack updates for now.
- Ensure that localization works in the GENERIC viewer, even if the necessary locale files cannot be loaded.
This was the behaviour prior to the introduction of Fluent, and it seems worthwhile to keep that (especially since we already bundle the en-US strings anyway).
- Let the `GenericL10n`-implementation use the *bundled* en-US strings directly when no language is provided.
- Remove the `NullL10n`-implementation, and simply fallback to `GenericL10n`, to reduce the maintenance burden of viewer-components localization.
- Indirectly, given the previous point, stop exporting `NullL10n` in the viewer-components since it's now removed.
Note that it was never really intended to be used directly and only existed as a fallback.
*Please note:* This doesn't affect the Firefox PDF Viewer, thanks to the use of import maps.
To prevent the *standalone* viewer-components from breaking, we need to ensure that the `NullL10n`-interface won't accidentally diverge from the actual `L10n`-implementations.
Currently this class contains a few "special" code-paths for the COMPONENTS build-target, which normally wouldn't be a problem. However, in this particular case that means accessing code that we don't want to include unconditionally in all builds.
This is currently implemented using build-time `require`-calls which we nowadays want to avoid, and we should strive to remove all such cases from the code-base. (Generally speaking `import` is the future, and build-tools may not always play well with a mix of both formats.)
We can easily improve things here by using sub-classing for the COMPONENTS build-target, and then use the ability to re-name when exporting (to avoid breaking existing code).
Until now we've not actually had *any* tests that ensure that the *official* PDF.js-viewer API exposes the intended functionality, which means that things can easily break accidentally.
*Please note:* This unit-test cannot (easily) be run in Node.js-environments, since the `external/webL10n/l10n.js` file contains various browser-specific functionality.
