Add classd snippet for creating a dataclass

[macintacos]

This adds the classd snippet, which creates a class definition using the @dataclass decorator, adds the necessary import statement, and provides a sample property for the user to fill in.

User can remove the import statement (or have their editor do it for them). I'm on the fence about leaving the import statement in, as folks new to Python might define multiple classes within the same file and have multiple import statements, but presumably they'll have a linter setup to save them. Up to you though.

[cstrap]

Thanks!