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.
* Always add `next` to block representation
* Add `shadow` property to inputs, to maintain obscured shadows
* Create obscured shadows in SB2 import
* Add XML import of obscured shadows
* Alias SB2 shadow inputs to block inputs
* Add shadow to inputs on "delete inputs" test
* Add a small test to ensure obscured shadows are preserved
* Add more obscured shadow tests
