[WIP] Add structure reading as a clipboard format.

[wizjany]

This is pulled from my original branch (https://github.com/EngineHub/WorldEdit/tree/feature/structures), but without the MultiClipboard mess.

Current non-goals of this implementation include supporting multiple palettes, jigsaw/data blocks, and writing.

TODOs:

• testing
• DFUing (that's new since the last branch)
• handle voided blocks (ie vecs within the bounds but no entry within the blocks tag)
• possibly a way (craftscript?) to extract the existing structures from the minecraft jar to the WE schematic dir.