ImJoy is now loaded dynamically, and can be turned off with
an environment variable.
I've added two new scripts to develop and build without ImJoy.
I suspect these won't see much use, but
they will remind us how to do it when we
want to release a medical version.
The ImJoy "chunk" is named for ease of separation.
Co-authored-by: Kaspar Emanuel <kaspar@monostable.co.uk>