Restore navigator.plugins and navigator.mimeTypes as PDF-focused

This addresses the discussion starting at whatwg#6003 (comment).

It ensures browsers only vary on a single boolean, "PDF viewer supported". Then, it:

* Standardizes a new navigator.pdfViewerSupported property which exposes that boolean directly.
* Makes the legacy navigator.plugins and navigator.mimeTypes APIs expose a uniform set of Plugin and MimeType objects, across every browser, in one of two configurations (depending on the PDF viewer supported boolean).
* Gets rid of the generic plugin path in navigation, instead replacing it with specific paths for application/pdf and text/pdf which are tied to the PDF viewer supported boolean.

Closes whatwg#6317 since that idea is no longer possible in this framework. Helps with whatwg#6003.