skin overrides

we want to get this mod out for 1.21.6 as soon as possible, so this version isn't the full release of 2.4.0, but comes with support for the new update and some small features (which will be improved in the full release of 2.4.0!)

• added support for Chase the Skies (1.21.6)
  • this might be a bit broken, so please report any issues!
• removed support for 1.15.2 - 1.19.3
• the cape library now has your capes by default
  • if you already have a cape library, nothing will happen
  • there will be a way to add these to an existing library in the future!
• fixed adding multiple entries to the library having the wrong texture (gh#26)
• fixed overrides not properly being removed on 1.19.4 - 1.20.4 (gh#26)