Changes include:
- Added missing JSDoc for items in `scratch3_pen.js` and `target.js`.
- Moved constants used by `Scratch3PenBlocks` into the class.
- Created a constant for minimum and maximum pen size.
- Added `util/clone.js` to host cloning functionality.
- Pen blocks now check for the renderer before trying to use it.
- The pen integration test covers all blocks, though `clear`, `stamp`,
and `pen down` will skip some of their functionality when there is no
renderer.
