Quality Control 🎖

• Introduces a new SpinKit [SpinKitSquareCircle]
• Introduces const SpinKits
• Improve code quality across all spinners

Before

final spinkit = SpinKitSquareCircle(color: Colors.white, size: 50.0);

Now, all SpinKits can be initialized as consts

const spinkit = SpinKitSquareCircle(color: Colors.white, size: 50.0);

Awesome #61 PR by @Abhishek0706