Improve accuracy of OpenAI capabilities supports_vision? and supports_functions? methods (#56)

This is still not perfect since OpenAI has lots of models, but it is
much better than what it used to have.

Also fixed a bug where RubyLLM.models.refresh! would return the old list of models, and refactored model handling in RubyLLM: delegate class methods, add save_models method, and streamline model loading

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Co-authored-by: Carmine Paolino <carmine@paolino.me>

Author: jaggdl