Add support for EM German Mistral Model

[jphme]

Describe your changes

Added the just-released EM German Mistral Model to models2.json.
You can find more information, example outputs and so on in the models repository. The model comes with the Apache-2.0 license of the base model.

TBH, I still don't understand 100% how GPT4ALL`s model list works (I suppose its still based on a proprietary hosted model list?); the model wasn't available for download in the model list when I rebuild GPT4all, but I confirmed that the model works falwlessly within the most recent app.

I asked in Discord and they said I should just prepare a PR.

Reson for this is I am the author of the most-downloaded llama-2 model finetuned on German texts and people kept asking me how to use it in a GUI; would be good to have a user-friendly alternative to LM Studio, thus I would appreciate support within GPT4ALL.

Checklist before requesting a review

• [X ] I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• I have added thorough documentation for my code.
• I have tagged PR with relevant project labels. I acknowledge that a PR without labels may be dismissed.
• If this PR addresses a bug, I have provided both a screenshot/video of the original bug and the working solution.