Forbid unknown fields - #210

>>> class Example(msgspec.Struct, forbid_unknown_fields=True):
...     field_one: int
...     field_two: bool = False

>>> msgspec.json.decode(
...     b'{"field_one": 1, "field_twoo": true}',  # oops, a typo
...     type=Example
... )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Object contains unknown field `field_twoo`