Currently, we distinguish between a Prompt interface and the prompt action.
In the JS, and now in the rest of the Go ai package, the former interfaces are now type aliases for actions.
Also, in the JS, the action of a prompt is to render it, not to generate text from it.

Rewrite the general prompt API to conform more closely to what the JS does (js/ai/src/prompt.ts).
The JS PromptAction is generic in its input, so we can't use a type alias; we will need a new generic type.
We should also change the action to render.