Support configuring entrypoint file in tspconfig.yaml

### Clear and concise description of the problem

Now we only support configuring entrypoint file in package.json using ```tspMain``` property if users don't want to use main.tsp for some reason. But when users have project using one package.json to manage multiple typespec projects (i.e. our specs repo), the config in single package.json won't help too much. People would still need to pass the entrypoint file explicitly when compiling. So in this case, it will be helpful if people can also configure the entrypoint file in tspconfig.yaml directly.

### 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.