Support customizing package.json in http-client-js

### Clear and concise description of the problem

I'm using a bun monorepo with workspaces and running into a small issue with generating code with typespec and the http-client-js emitter.

The package.json generates the following inside the package.json.

```
"exports": {
".": "./dist/index.js",
"./models": "./dist/models/index.js"
}
```

However, since bun can execute TS directly I don't need to run `tsc` and can reference the generated TS directly.

Are there ideas on how to support customizing the package.json so I could change this to the following?

```
"exports": {
".": "./src/index.ts",
"./models": "./src/models/index.ts"
}
```

Is there an alternative recommended way to support this? I could probably just add an extra build step to compile just this package to keep it simpler for now without hacking the generated package.json.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.