Convert `PDFDocument._getLinearizationPage` to an async method
Convert `PDFDocument._getLinearizationPage` to an async method

This, ever so slightly, simplifies the code and reduces overall indentation.

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