Loading blockexchange files

[S-S-X]

Optionally allow loading simple worldedit blockexchange files to be used as test world.

Currently test world must be either:

• written with world.layout / world.add_layout
• or created using minetest.set_node
• or nodes placed by Player:do_place

All of this requires writing code to build test world, using worledit blockexchange files would be nice as you could export pieces for texting from game.
And it would also allow way easier initial param2 / metadata settings for test world.

[S-S-X]

Worldedit is AGPL-3.0 which is way too restrictive and therefore Worldedit interface cannot be easily implemented by using existing libraries.

There's however https://github.com/blockexchange/blockexchange and comes with MIT license, maybe forget about Worldedit and take a closer look at Blockexchange (also seems like it is using mineunit for tests 😄)