Convert the `PDFPageProxy.intentStates` property from an `Object` to a `Map`
Convert the `PDFPageProxy.intentStates` property from an `Object` to a `Map`

As can be seen in the code there's a handful of places where this structure needs to be iterated, something that becomes cumbersome when dealing with `Object`s. Hence, by changing this to a `Map` instead we can both simplify the code and avoid creating unnecessary closures.

Particularily the `PDFPageProxy._tryCleanup` method becomes a lot more readable, at least in my opinion.

Finally, since this property is intended to be "private" the name is adjusted to reflect that.

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