Postjam Cleanup version release

Added new standalone version with some minor fixes / improvements / quality of life stuff:

• Imported some CC0 replacement assets for the vehicles, grass, and checkpoints. Redesigned finish line as well.
• Prevented player from driving out of bounds.
• Added simple labels to the icons on the upgrade menu.
• Added race result label at the end of each race. 
• Slightly easier gameplay  due to smaller car models
• Added splash and icon