feat: support other LLMs

[declann]

Using VizGPT sends data to Microsoft/OpenAI & uses proprietary GPT model.
The possibility to use other LLMs instead would be nice. Free/open source and local & fine-tuned models could open up other usecases and be a huge improvement.

[declann]

I tried to get some pure vega-lite specs/manipulation out of Star Coder and the results aren't very good. It likes to give Python code using Altair however!

https://huggingface.co/spaces/bigcode/bigcode-playground

Better prompt/fine tuning work might make this good however.

Other LLMs to try aso e.g. from Salesforce and Replit (out of assistant-type LLMs)