tsp-openapi3 - add support for generating tsp config files

### Clear and concise description of the problem

Related to #3663

The tsp-openapi3 cli currently generates a `main.tsp` file from an OpenAPI3 spec. It would be helpful to optionally generate all the project files (package.json, tspconfig) so that all dependencies are immediately available for new projects.

As a workaround, today a user can run `tsp init` to generate an empty project, then run `tsp-openapi3` to generate the `main.tsp` file in that project.

### 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.