Extension Versioning

[pmalacho-mit]

Given we want to support active extension development (meaning folks are updating blocks regularly, etc.), we need to introduce some kind of versioning system to extension blocks + saved scratch projects (.sb3 files -- which are really just zip files containing a json file describing the blocks in use, as well as any saved media, e.g. sounds, images).

The current scratch serialization system (or at least the copy present in this codebase) is very fragile to block definition changes.