Simplify handling of `requestFullscreen` errors in `PDFPresentationMode`
Simplify handling of `requestFullscreen` errors in `PDFPresentationMode`

Since quite some time the `Element.requestFullscreen()` method has been returning a Promise, see https://developer.mozilla.org/en-US/docs/Web/API/Element/requestFullScreen#return_value
Hence we can utilize that to detect failures to enter fullscreen-mode, and remove our old `setTimeout`-based hacks that were used for this purpose.

According to the MDN compatibility data, see https://developer.mozilla.org/en-US/docs/Web/API/Element/requestFullScreen#browser_compatibility, all browsers that we support have implemented this functionality. (Note that after PR 14606, we no longer support PresentationMode in Safari.)

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