Quaternion - Rewrite .slerp() and .slerpFlat()

[WestLangley]

This PR modifies Quaternion.slerp() and Quaternion.slerpFlat() to use the same algorithm -- the one used by Unity and Filament.

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True spherical linear interpolation is unnecessary for small angles. Instead, linear interpolation followed by a re-normalization is used for small angles.

I noticed in the FBX and Collada skinning examples, the small-angle case occurs less than 2% of the time.

When switching to the algorithm used by Unity, the same skinning examples execute the small-angle case about 75% of the time.

The lesson here is to avoid using Number.EPSILON indiscriminately.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.24 79.14	338.24 79.14	+0 B +0 B
WebGPU	580.27 160.28	580.27 160.28	+0 B +0 B
WebGPU Nodes	578.88 160.04	578.88 160.04	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	469.88 113.85	469.7 113.74	-177 B -104 B
WebGPU	649.98 175.93	649.8 175.85	-177 B -85 B
WebGPU Nodes	604.09 165.11	603.91 165.01	-177 B -97 B

[Mugen87]

Does that mean the new approach enables performance improvements (probably depending on the input data)?

[WestLangley]

Does that mean the new approach enables performance improvements (probably depending on the input data)?

Correct.