Support generator command actions

[Sebobo]

This would allow a command to respond with multiple messages while its working. Also a command might allow user input and use it for further processing.

• [x] Support async generators
• [x] React to yielded values and show messages
• [ ] Show rich input if available
• [x] Use it for an actual feature

[Sebobo]

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[Sebobo]

Remaining task is done in #5