Summary

Add option for picking PDFs on all supported platforms. I realize we could use a FilePickerFileType for all possible filetypes (or maybe a parser), but PDFs is such a common format that i think it justifies the addition.

API Changes

To make it simpler for developers to pick PDFs we add FilePickerFileType.Pdf to the FilePicker API for all supported platforms.

Intended Use Case

Picking PDFs