Convert `PartialEvaluator.buildToUnicode` to an `async` method
Convert `PartialEvaluator.buildToUnicode` to an `async` method

This removes the need to *manually* wrap all return values in a Promise.

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