add "tag-of" to Post Type Algorithm after sufficient publishing evidence

[tantek]

Once there is sufficient publishing evidence for tag replies (u-tag-of in the brainstorm), it should be added to the response and full PTD algorithms.

Note the AP/AS2 request for explicit type for (hash)tag w3c/activitypub#235 - likely an indication that PTD should handle discovery and output of this as well.

https://indieweb.org/tag-reply

[tantek]

Insufficient implementation to add at this time. Postponing til after published as a W3C NOTE.

[snarfed]

bridgy will soon consume tag-of posts and need/use it in post type discovery: snarfed/bridgy#811