Update C# New Project template to include graphics example

[anandika-nair]

Description

This pull request updates the default C# New Project starter template in SplashKit Online.
Previously, the C# starter project only demonstrated SplashKit.WriteLine. This change replaces it with a simple graphics-based example that opens a window and draws a basic house scene using SplashKit functions such as OpenWindow, ClearScreen, FillRectangle, FillTriangle, and RefreshScreen.
This makes the C# starter project more beginner-friendly and gives new users an example that introduces SplashKit graphics.

How Has This Been Tested?

I tested the change locally by running SplashKit Online using:

npm start

## Checklist
- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my own code
- [ ] I have commented my code in hard-to-understand areas
- [ ] My changes generate no new warnings
- [ ] I have requested a review from ... on the Pull Request

## Notes
This change updates the default C# starter template only. 
Full execution of C# graphics in SplashKit Online depends on current runtime support, which is limited and outside the scope of this PR.