Pack migration

[DeflatedPickle]

There are multiple formats for packs. It would be nice to be able to migrate a pack forwards or backwards to a version, with checkboxes to remove unused textures or add missing textures from the version JAR

It would be annoying to write migrations between every version, so it might be easier to write migrators that can be chained

To-Do:

• phantom for 1.8/1.12 -> 1.13/1.15
  • Resource Pack Converter (check everything in phantom is fine against this)
• unstitcher for <1.4? -> 1.5
  • Wait for license
• TextureEnder for texture pack -> resource pack
  • Wait for license
• slicer for ?.? -> 1.14