Build base classes and visitors for custom shapes in p5.js 2.0

[GregStanton]

Addresses #6560

Changes:

• Build base classes (for shapes, contours, shape primitives, and vertices)
• Build visitor classes (for converting shape primitives to Path2D objects or vertices)

Notes:

1. This PR may also include an implementation of a point-at-length getter for path-segment primitives.
2. This PR includes boilerplate for the new user-facing functions; that's a roadmap for a later PR, so it's commented out.

PR Checklist

Note: Since this is a draft PR, I'll update the checklist before submitting it for review. Inline documentation will be included, but I'll hold off on including unit tests covering the new behaviors, since those won't be testable until the next phase of work is completed.

• npm run lint passes
• Inline documentation is included / updated