TSL: Fix outgoingLight redeclaration

[sunag]

Description

.toVar() adds the node to the current Stack, if this is called improperly it can cause redeclaration, as was the case when the class LightsNode was declared.

[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	553.97 153.41	554.01 153.42	+41 B +7 B
WebGPU Nodes	553.32 153.26	553.36 153.26	+41 B +7 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	629.59 170.31	629.63 170.32	+41 B +7 B
WebGPU Nodes	584.44 159.66	584.48 159.67	+41 B +7 B