Use installed SplashKit dependencies

[SEBINSHASAMEERSHA]

Summary

• remove the direct external json.h include
• use the installed SplashKit header through <splashkit.h>
• remove the outdated bundled SplashKit headers
• update VS Code include paths to stop referencing the removed headers

Testing

• built successfully with: skm clang++ *.cpp -o DXBallGame-test
• launched DXBallGame-test.exe successfully
• confirmed the game runs normally

[jankiluitel]

Thanks for making this update. Moving the project to use the installed SplashKit dependencies instead of maintaining a local copy is a good improvement, as it reduces duplicated files and makes the project easier to maintain.

I have one suggestion before this is merged:

Since this PR removes the entire include/splashkit directory, it would be helpful to confirm that the project has been built successfully on the supported platforms (Windows, macOS, and Linux if applicable). This change affects the project's build configuration, so verifying that a clean checkout still compiles using an installed SplashKit environment would provide confidence that no include paths or configuration issues remain.

Other than that, the include updates look consistent with the goal of the PR.