Add Jest Testing Framework and Initial Test Suite

[elizasviel]

This PR adds a comprehensive testing framework to the Phaser codebase using Jest, along with an initial set of unit tests for core components.

• Set up Jest as the testing framework with appropriate configuration
• Added test scripts to package.json (test, test:watch, test:coverage)
• Created a mock environment for browser APIs to enable testing browser-dependent code
• Implemented unit tests for several core components:
• Math: Vector2, angle utilities, RandomDataGenerator
• Geometry: Point, Rectangle
• Core: Basic TimeStep tests

[cryonautlex]

I've found five bugs in Phaser since 2025-03-18. And then I find this, today: @zekeatchan disapproves of this golden opportunity to start adding unit-tests to Phaser. I'm losing faith in the maintainers, pure and simple.