* Add `getTargetIdForDrawableId` to translate between renderer picks and VM targets
* Add `startDrag` to stop sprite motion while dragging
* Add `stopDrag` to return sprite motion after dragging
* Move Node output: /dist.js => /dist/node/scratch-vm.js
* Move web output: /vm{.js,.min.js} => /dist/web/scratch-vm{.js,.min.js}
* Update build output references in package.json and the playground's index.html
* Move the VirtualMachine class out of index.js into its own file, referenced by index.js. The VirtualMachine class is otherwise unchanged.
* Add .gitattributes rules for new file types which were added to this repository without specifying their text/binary attributes
* Turn on source maps in webpack and add corresponding .gitignore rule
