This MR refactors registries (previously `BaseRegistry`, now just `Registry`)
to contain entries based on `DataModel`s.
With that it gets rid of some of the pre-`DataModel` handling
(including `.data`).
Initialisation of registries is split into two parts:
1. The self-contained `_initialize` (which should allow future `DataModel`
  implementations for documents to refer to them for their `choices`)
2. A `setup` step run in the `setup` hook, allowing registry entries to
  localise their own fields, replacing translation keys

API documentation for registries (including an example for `DamageType`
addition) is added.

It includes **breaking changes**:
- The previously forgotten `pf1.register<Registry>` hook is immediately replaced
  with the now-common format of `pf1Register<Registry>`.
  Its second parameter (the entry class) has been removed, as it is available
  through `registry.module` should a caller need it – which should usually not
  be necessary due to `Registry#register` taking a data object.
- `DamageType` entries have their modifier boolean moved from a flag to the
  actual object, now using `isModifier`.
- `module` fields are replaced with a more general `namespace`.