[JS] Override the `Doc.external`-getter to avoid `alert`-modals on load (issue 15509)
[JS] Override the `Doc.external`-getter to avoid `alert`-modals on load (issue 15509)

This property is documented in https://web.archive.org/web/20201112021418if_/https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/AcrobatDC_js_api_reference.pdf#G5.1977075

Given that PR 14207, which is *somewhat* similar, landed without tests that's hopefully fine here as well.

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