Rework tweening system and fix tweening bugs

[stringdotjar]

Description

closes #47
closes #59

This pull request fixes the cancel() crash, and also reworks the way tweens are added to the global (or any) FlixelTweenManager. It also reworks the tweening system to use default builders for constructing tweens with ease, while also leaving the old methods for simpler use.

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)

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