fix: draggable interface and concrete dragger

[BeksOmega]

The basics

• I validated my changes

The details

Resolves

Fixes N/A

Proposed Changes + reasons

This refactors the implementation to match the agreed upon design with two changes:

1. Draggables won't take in IDragTarget objects. I don't know why I did that, since the dragger handles all interaction with drag targets.
2. We have a new moveToStart method that is (essentially) used to cancel a drag in the case that the IDragTarget returns true for shouldPreventMove. Before we were just moving the block back to the original coordinates, but that doesn't allow blocks to reattach to parents or children.

The agreed upon design has a good separation of concerns between the draggable and the dragger. Giving the draggable responsibility over handling drag targets confuses that, and breaks the single responsibility principle. It also makes it more likely for an external developer to handle the drag targets incorrectly, because there is some fiddliness about the order methods shouldb e called in.

Test Coverage

Manually tested this with following PRs.

Documentation

Yes this will all need docs. May end up happening after the v11 release though =(

Additional Information

N/A