[http-server-csharp] Default folder names to Pascal casing, not lower

### Clear and concise description of the problem

Right now all folder names are lowercase. Most c# projects I've worked on use Pascal case for folder names. It would be great to follow that convention.


### 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.