R1:1b96f2fc2870

Add `maniphest.subtypes` for configuring task subtypes

Summary:
Ref T12314. Builds toward letting you define "animal" and "plant" tasks.

This just adds some configuration. I'll probably add some more quality-of-life options (like "icon") later but these are the only bits I'm sure I'll need.

Test Plan:
- Configured sensible subtypes.
- Tried to configure bad subtypes: bad key, missing "default", duplicate keys. Got sensible error messages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314…