DeepSeek's Expert mode

[mcendu]

With DeepSeek v4 they actually got two models, codenamed deepseek-v4-flash and deepseek-v4-pro and respectively termed "Instant Mode" and "Expert Mode" on the web interface.

I guess we can add one more menu option for the DeepSeek provider. The current option could be renamed "DeepSeek Instant" (the coder variant was dropped v2.5), and a new menu entry "DeepSeek Expert" could be added.