Fix assignments to properties of inputs in branches in p5.strands

[davepagurek]

Resolves #8396

Changes:

• Previously, we would call .copy() on any object modified in any way in a branch. If you assign to color.rgb, we want to do color.copy(). That made sense because that's a sizzle assignment. But it broke if you assigned to inputs.color, it would do inputs.copy(), which doesn't work because inputs is a struct.
• Now, we copy the outermost property accessed object that is not a swizzle. So that's color in color.rgb, inputs.color in inputs.color, and inputs.color in inputs.color.rgb.

Live: https://editor.p5js.org/davepagurek/sketches/vOrLgKr0R

PR Checklist

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