tsp install should add emitters to tspconfig.yaml

### Clear and concise description of the problem

When you `tsp install` an emitter package, it should add it to tspconfig.yaml either by default, or prompt the user if they want to do that. We would need a way to detect emitter packages, which might be possible by importing it after install and seeing if it has an exported `$onEmit`.

### 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.