Use standard `import` statements when loading `PDFViewerApplication`/`AppOptions` in `web/viewer`
Use standard `import` statements when loading `PDFViewerApplication`/`AppOptions` in `web/viewer`

Given that we're no longer using SystemJS to load the `web/` files, see PR 11919, there's nothing that prevents us from using standard `ìmport` statements in this file.
Obviously it's still necessary to load part of the code conditionally on the build type, however this still allows us to clean-up and simplify at least some of this file.

timvandermeij/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,macroplant/pdf.js,Shoobx/pdf.js,xavier114fch/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,macroplant/pdf.js,timvandermeij/pdf.js,timvandermeij/pdf.js,xavier114fch/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,caremerge/pdf.js,Shoobx/pdf.js,caremerge/pdf.js,nawawi/pdf.js,macroplant/pdf.js