[feature] provide examples for a prompt/response

[cdreetz]

Currently this works

mix = ["san antonio, tx", "marfa, tx"]

ds = dataset({
    "id": sample.uuid(),
    "task": sample.choice(mix),
    "prompt": gen("write a prompt asking about the history of {task}"),
    "response": gen("write a response to {prompt}"),
}, n=2)

but I would also like to be able to do something like

mix = ["san antonio, tx", "marfa, tx"]

ds = dataset({
    "id": sample.uuid(),
    "task": sample.choice(mix),
    "prompt": gen("write a prompt asking about the history of {task}", example="What is the significance of the Alamo in San Antonio?"),
    "response": gen("write a response to {prompt}"),
}, n=2)