Rework tweening system and fix tweening bugs

[stringdotjar]

Description

closes #47 and closes #45

This pull request fixes the cancel() crash, and also reworks the way tweens are added to the global (or any) FlixelTweenManager.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor / Optimization

Checklist

• My PR targets the develop branch, not master/main.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes pass all automated build checks.
• I have updated the documentation accordingly (if applicable).
• I have added tests that prove my fix is effective or that my feature works.

Screenshots / Evidence (if applicable)

Add screenshots or logs to help explain your changes.