Adds generateText and generateOutput convenience methods.

[mbleigh]

This adds new generateText and generateOutput convenience methods that return like so:

const {text, response} = await generateText({prompt: "Tell me a joke"});

const {output, response} = await generateOutput({
  prompt: "Create a creature",
  output: {
    schema: z.object({
      name: z.string(),
      hitPoints: z.number()
    })
  }
});

It's a small ergonomic change but it prevents needing to assign the response to a variable if you don't care about it.

If you like the idea we can talk about how we'd want to document it vs. other options.

[apascal07]

@mbleigh I saw you added these things to the 0.6 release doc. Did you want to submit this PR?