extensions: import the exthelper class from evolve 980565468003 (API)
This should help make extensions that wrap a lot of stuff more comprehendible.
It was copied unmodified, except:
- fix up the imports
- rename final_xxxsetup() -> finalxxxsetup() to appease checkcode
- avoid a  default arg to wrapcommand()
Add `exthelper` class to simplify extension writing by allowing functions, commands, and configitems to be registered via annotations. The previous APIs are still available for use.