Chat with multiple LLMs

[basicthinker]

Is your feature request related to a problem? Please describe.

We often want to chat with multiple LLMs and compose the best solution. This can be an attractive unique feature of DevChat.

Describe the solution you'd like

1. Enable selecting multiple LLMs for chat and show their names.
2. The chat panel embed a tab view showing multiple LLM replies.
3. Internally, devchat is called in parallel with selected LLMs, storing their chat histories in sperate git branches.