Add logfire.warning to mirror stdlib logging.warning

[JacobHayes]

Description

The standard library's logging module uses warning and deprecated warn. It'd be helpful to at least make warning available as logfire.warning and logfire.Logfire.warning.

This would make it a bit easier to write code that can accept either a logging.Logger or a logfire.Logfire instance (eg: library code or easier test injection).

I'm happy to submit a PR for this if you all are open to this. I see a prior commit replacing the usage of a warning method with warn, but I think the referenced PR was from a prior repo / before it was public, so I'm not sure if warning was removed intentional before.

[alexmojaki]

This is a good point, a PR would be great, thanks!