Feature: Add MDN URLS

[Bashamega]

This issue will require significant effort, but it's for a great cause.

We currently have a file called addedTypes.json, which contains manually added types. Some of these types already include an mdn_url property, but many do not.

Adding MDN URLs for all entries would be very beneficial — it would help users learn more about each API and also it would automatically allow the description generator to generate comments for them, especially after #1993 gets merged.