Simplify the `DocumentInfoValidators` definition
Simplify the `DocumentInfoValidators` definition

With this file now being a proper (ES6) module, it's no longer (technically) necessary for this structure to be lazily initialized. Considering its size, and simplicity, I therefore cannot see the harm in letting `DocumentInfoValidators` just be simple Object instead.

While I'm not aware of any bugs caused by the current code, it cannot hurt to add an `isDict` check in `PDFDocument.documentInfo` (since the current code assumes that `infoDict` being defined implies it also being a Dictionary).

Finally, the patch also converts a couple of `var` to `let`/`const`.

mainegreen/pdf.js,nawawi/pdf.js,caremerge/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,mukulmishra18/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,xavier114fch/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,Snuffleupagus/pdf.js,caremerge/pdf.js,mozilla/pdf.js,mozilla/pdf.js,nawawi/pdf.js,macroplant/pdf.js,macroplant/pdf.js,Shoobx/pdf.js,Shoobx/pdf.js,mainegreen/pdf.js,mukulmishra18/pdf.js,xavier114fch/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,showpad/mozilla-pdf.js