Fix Collision Bug and Add STEM Distance Block

[brettfxio]

This PR addresses a bug in the physics collision system where only the first registered callback for a specific pair of colliding objects would fire. It also introduces a new STEM-focused block, get_distance, which allows users to measure the distance between any two named objects in the 3D scene.

Key changes:

1. Collision Logic: Updated onCollision to include a unique callbackId in the collision tracking key, allowing multiple independent collision handlers to coexist.
2. STEM Enhancements: Added getDistance method to BabylonSceneManager and integrated it into a new Blockly block.
3. UI Improvements: Renamed the 'Utils' category to 'Math' to better reflect its purpose and included the new distance block there.
4. Reliability: Enhanced cleanup in destroyObject and clear to prevent stale references in the render loop.
5. Testing: Added and executed Playwright tests to verify the fixes and the new feature.

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PR created automatically by Jules for task 2155587819738579984 started by @brettfxio

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