TSL: Move MatcapUVNode to matcapUV Fn constant

[sunag]

Description

Simplification. Move MatcapUVNode to matcapUV Fn constant.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.54 78.73	337.54 78.73	+0 B +0 B
WebGPU	556.43 154.15	556.36 154.14	-64 B -7 B
WebGPU Nodes	555.78 153.99	555.71 153.98	-64 B -6 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	468.74 113.38	468.74 113.38	+0 B +0 B
WebGPU	632.24 171.08	632.19 171.09	-45 B +7 B
WebGPU Nodes	587.24 160.39	587.24 160.39	+0 B +0 B