* Add `Clone.prototype.getCostumeIndexByName`, keep in range
* Add basic costume primitives from Scratch 2.0
* Add costume getter block
* Add properties and methods for distinguishing stage and sprites-vs-clones
* Add backdrop-related looks blocks
* Fix up "switch to backdrop" to be working
* Costume/backdrop reporters are 1-indexed
* Fire "when backdrop switched" hats
* Cut cloning helpers for a separate PR
* Disable many blocks on the stage
* Refactor into _setCostumeOrBackdrop; implement switch backdrop and wait
* Fire hats even when backdrop unchanged