Support json-schema generation

It would be helpful if JSON schema generation was supported

```python
class User(msgspec.Struct):
    """A new type describing a User"""
    name: str
    groups: Set[str] = set()
    email: Optional[str] = None

schema = User.json_schema()
```

Similar to the functionality seen in https://github.com/s-knibbs/dataclasses-jsonschema