feat: make ai.definePrompt callable as a function

[pavelgj]

const hi = ai.definePrompt({
    name: 'hi',
    model: gemini15Flash,
    input: {
      schema: z.object({
        name: z.string(),
      }),
    },
  },
  'hi {{ name }}'
);

const response = await hi({ name: 'Genkit' });
console.log(response.text())

// or stream

const { response, stream } = await hi.stream({ name: 'Genkit' });
for await (const chunk of stream) {
  console.log(chunk.text());
}
const responseText = (await response).text();
console.log(response.text())

or

const hi = ai.definePrompt({
    name: 'hi',
    model: 'echoModel',
    input: {
      schema: z.object({
        name: z.string(),
      }),
    },
    config: {
      temperature: 11,
    },
  },
  async (input) => {
    return {
      messages: [
        { role: 'user', content: [{ text: `hi ${input.name}` }] },
      ],
    };
  }
);
      
const response = await hi({ name: 'Genkit' });
console.log(response.text())

// or stream

const { response, stream } = await hi.stream({ name: 'Genkit' });
for await (const chunk of stream) {
  console.log(chunk.text());
}
const responseText = (await response).text();
console.log(response.text())