Add detection for outside variable references in p5.strands uniforms

[Aayushdev18]

Resolves #8172

Changes:

• Added detection for outside variable references in p5.strands uniform functions
• Implemented simple string checking for common problematic variables (mouseX, windowWidth, state_, pixelate)
• Integrated error detection into the existing setDefaultUniforms() method in p5.Shader.js
• Added helpful error message using p5._friendlyError() when outside variables are referenced

The implementation detects when users reference variables not defined within the shader's scope in p5.strands callbacks and provides clear feedback, improving the debugging experience for users working with shader uniforms.

Screenshots of the change:

N/A - This is a backend error detection feature

PR Checklist

• npm run lint passes
• [Inline reference] is included / updated
• [Unit tests] are included / updated

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