[Feature] Manual chunks

Similar to Rollup's [`manualChunks`](https://rollupjs.org/guide/en/#outputmanualchunks) option, where you can define custom common chunks

```json
{
"manualChunks": {
"common": ["react", "react-dom"]
}
}
```

Start by reading the linked Rollup manualChunks documentation and compare its behavior with esbuild's current chunking options. Define how the shown manualChunks mapping should affect the generated bundles and verify the result with a small React and react-dom example. Done means named common chunks are produced according to the configuration.

Written by the indexing model from the issue text.