Add instanceIndex alias for instanceID

[aashu2006]

Addresses #8911

Changes

• Add instanceIndex as a value-based API for accessing the current instance index in p5.strands
• Preserve instanceID() for backwards compatibility
• Update docs and examples to use instanceIndex as the preferred API
• Update WebGL and WebGPU tests to use instanceIndex

instanceIndex is exposed as a property rather than a function, making it feel more consistent with other strands values such as width, height, and mouseX. The existing instanceID() API remains available for compatibility.

PR Checklist

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

[aashu2006]

@davepagurek I have updated it based on the discussion around instanceIndex.

instanceIndex is now exposed as a value instead of a function, while instanceID() remains unchanged for compatibility. The implementation uses a dedicated property accessor that creates the same underlying node as instanceID(), and docs/examples have been updated to use the value-based API.