Make grammar initializer prettier

[crimson-knight]

Currently grammars use from_json()

This is not as pretty or convenient as new.
Maybe a macro for defining properties would work better and could write the initializer.

[crimson-knight]

A cleaner implementation will allow provider just the class and then handles creating the instance to return, like the following

# The `response` here is now an instance of MyStructuredResponse
response = DEFAULT_MODEL.chat(prompt, grammar_class: MyStructuredResponse)