Add drawStar() function to draw customizable star shapes

[XTC917]

Changes:

• Added a new drawing function star(x, y, radius1, radius2, npoints) to p5.prototype in src/core/shape/2d_primitives.js.
• This function allows users to draw star-shaped figures by specifying outer radius, inner radius, and the number of points.
• Also added demo files: test-star.html and test-star-interactive.html to illustrate basic and interactive usage.

Description:
I found that the project didn’t have a clearly documented or easily accessible star-drawing function, so I contributed p5.prototype.star, located around line 1490-1517 in src/core/shape/2d_primitives.js.

Implementation details:

• Parameters:
  x, y — center position of the star
  radius1 — outer radius
  radius2 — inner radius
  npoints — number of star points (default 5)
• Steps:
  1. Start with beginShape().
  2. Calculate angle and halfAngle for spacing star points evenly.
  3. In a loop, alternate between outer and inner points using radius1 and radius2.
  4. End with endShape(CLOSE) to complete the star.

This function enhances the creative capabilities of p5.js, especially for users working in visual arts and design.

Screenshots of the change:

(Add screenshots here if available, e.g. result of test-star.html)

PR Checklist

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

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