feature enable sets for 12y2 variants

[y-ack]

• 12y2[-c]: ('convenience for chat')
  • ALL ALLOW
• 12y2-d: ('predictable for documents')
  • DISABLE convenience style syntax:
    • **
    • __
    • ~~
    • /
  • ALLOW tags
  • ALLOW heading bol #, ## etc.
  • ALLOW divider bol ---
  • ALLOW quote bol >
  • ALLOW inline code `
  • ALLOW code bol ```
  • ALLOW url
  • ALLOW embed !url
  • ALLOW link
  • ALLOW table bol |

a request was made for

• 12y2-p: ('pedantic')
  • ALL DISABLE
  • ALLOW tags

this needs #11 and is less useful for e.g. snail but may be a good compromise for 'i don't want to activate /anything/ on accident'

[y-ack]

...actually does that even make sense... i don't know anymore...
-c no longer has any of the accident-prone stuff anyway, and even the new block-closing rules make runaway tags like barely a problem...