Quaternion: Update epsilon in setFromUnitVectors().

[Mugen87]

Fixed #31286.

Description

Updates the epsilon value like discussion in #31286.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.57 78.74	337.57 78.74	+0 B +0 B
WebGPU	556.8 154.16	556.8 154.16	+0 B +0 B
WebGPU Nodes	556.15 154	556.15 154	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	468.76 113.39	468.75 113.42	-10 B +33 B
WebGPU	632.55 171.18	632.54 171.18	-10 B +0 B
WebGPU Nodes	587.6 160.53	587.59 160.53	-10 B -1 B