Add "constructing classes without new" for discussion.

I'm being bold, and submitting a pull request for something I think TC39 should briefly discuss! I'm not on the committee and nobody has specifically stepped up to champion this, but I thought I'd try leveraging the power of the new, democratic, GitHub-ized agenda.

Feel free to shut down this pull request mercilessly; I'll understand!

/cc @rwldrn in case he wants to champion it.