TSL: Improve Fn() warning if not invoked.

[sunag]

Related issue: #30849

Description

It's very much like using .colorNode = Fn( ... )(), but I often see users — myself included — forget to call it, ending up with a low-level message that doesn't say much. This PR improves the message, guiding the user to invoke the declared function.

material.colorNode = Fn( () => {

	return float();

} ); // missing () in the final

New message

Old message

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.84 79.07	338.84 79.07	+0 B +0 B
WebGPU	566.16 156.5	566.64 156.62	+481 B +115 B
WebGPU Nodes	564.77 156.26	565.25 156.37	+481 B +112 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.26 113.77	470.26 113.77	+0 B +0 B
WebGPU	637.36 172.49	637.84 172.63	+481 B +131 B
WebGPU Nodes	592.01 161.73	592.49 161.85	+481 B +120 B