Simplify the `PDFViewerApplication.supportsFullscreen` getter
Simplify the `PDFViewerApplication.supportsFullscreen` getter

A lot of the code in this getter has existed ever since the initial PresentationMode-implementation was first added all the way back in PR 1938 (which is nine years ago now).
At this point in time however, there's now a simpler way detect if a browser supports the FullScreen API and we should thus be able to simplify this getter; please refer to https://developer.mozilla.org/en-US/docs/Web/API/Document/fullscreenEnabled#browser_compatibility

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