Requests for comment/Parser function and tag tracking

We would like a standardized way to track the usage of parser functions and tags.

Background
Jack will fill this out

Problem
There are currently two main ways to do this: tracking categories or page properties


 * Extension:Graph uses w:Category:Pages with graphs
 * Extension:Kartographer uses w:Category:Pages with maps
 * Extension:Score uses a score page property

Some other extensions can also be tracked via page properties because they set them for every invocation, such as TemplateData.

Proposal
Automatically set a page property with the name of the function hook/tag when it is used.

Extensions could opt-out of tracking with a new  if they are already providing a different way to find usage. And we would introduce a similar constant for tag hooks.