Alias GLSL's mix function as lerp in p5.strands (#7875)

[LalitNarayanYadav]

Resolves #7875

Changes:

• Introduced an alias for the GLSL mix function under the name lerp in p5.strands.
• Ensures that lerp() within p5.strands maps to mix() when generating shader expressions.
• Falls back to the original p5.js lerp() function outside of p5.strands.

Rationale:

This change bridges the gap between core p5.js and p5.strands by making lerp() work seamlessly within shader code. It increases accessibility by allowing familiar p5.js functions to be used in new contexts.

A short inline comment was also added for clarity:

PR Checklist

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

[LalitNarayanYadav]

fixes #7875