TSL: Show an error message if no stack defined for assign operation

[sunag]

Closes #31748

Description

Show an error message if no stack defined for assign operation.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.11	338.91 79.11	+0 B +0 B
WebGPU	578.23 159.27	578.34 159.33	+116 B +55 B
WebGPU Nodes	576.83 159.03	576.95 159.08	+116 B +55 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.78 113.9	470.78 113.9	+0 B +0 B
WebGPU	648.76 175.3	648.88 175.35	+116 B +51 B
WebGPU Nodes	602.86 164.43	602.97 164.48	+116 B +51 B