Renaming the field `singletons` to `type_store` fixes the following
error in Zig master (0.10.0-dev.193+1d55e4cae):
`error: type '.ecs.type_store.TypeStore' not a function`
Returning a pointer makes the API a bit more ergonomic to consume,
allowing multiple calls to add and get to the singletons. For example:
```zig
var singletons = registry.singletons();
singletons.add(State.init(allocator));
var state = singletons.get(State);
```
a294babecc