R1:03b6bdde1931

Begin modularizing config options in a more modern way

Summary:
Ref T12845. Config options are "modular", but the modularity is very old, half-implemented, and doesn't use modern patterns.

Half the types are hard-coded, while half the types are semi-modular but in a weird hacky way where you prefix the type with `custom:...`.

The actual API is also weird and requires types to return a lot of `array($stuff, $thing, $other_thing, $more_stuff)` sorts of tuples.

Instead:

- Add a new replacement layer…