TSL: Add error handling for invalid shader code in FunctionNode

[Karthikeya1500]

Related issue: None

Description

This PR adds a missing error check inside FunctionNode.getNodeFunction().
When builder.parser.parseFunction() fails to parse the WGSL/GLSL shader
function, it returns undefined. This leads to unclear runtime errors later
in the node-building pipeline (e.g., accessing .inputs, .type, .name on
an undefined object).

By validating the parser result immediately and throwing a clear error
message, developers receive instant feedback about invalid shader code, making
TSL debugging significantly easier.

What this PR changes

• Adds a guard check after parsing a function:

  if ( ! nodeFunction ) {
      throw new Error(
          'TSL: Failed to parse function in FunctionNode. Check the shader syntax:\n' + this.code
      );
  }

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	356.24 86.48	356.24 86.48	+0 B +0 B
WebGPU	615.97 172.86	616.13 172.91	+160 B +51 B
WebGPU Nodes	614.57 172.61	614.73 172.66	+160 B +51 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	488.12 121.26	488.12 121.26	+0 B +0 B
WebGPU	688.33 188.85	688.49 188.91	+160 B +60 B
WebGPU Nodes	630.05 172.08	630.21 172.13	+160 B +56 B