feat: repackage genkit under genkit package and move CLI to genkit-cli

[pavelgj]

import { genkit } from 'genkit';
import { gemini15Flash, googleAI } from '@genkit-ai/googleai';

const ai = genkit({ plugins: [googleAI()] });

await ai.generate({
  model: gemini15Flash,
  prompt: 'hi',
});