base art pack no longer available on dropbox

[Ithamar]

The included URL to dropbox for the default artpack is no longer working. This also breaks the voxel-example, since that's using the default artpack.

Tried using
https://github.com/deathcap/ProgrammerArt/releases/download/v3.0/ProgrammerArt-v3.0-ResourcePack-MC19.zip but that has CORS issues :(

[deathcap]

Updated to GitHub URL in #16

To avoid the CORS issues, one can bundle the zip in the project and override the artpacks URL to a local path, such as in the (updated) voxel-example: voxel/voxel-example@6c34f67 (and how I was doing already in voxelmetaverse: https://github.com/voxel/voxelmetaverse/blob/98ade91bafe4a1a4e612fca2cf8169d6545003a5/index.js#L112-L114):

    'voxel-stitch': {
      artpacks: ['ProgrammerArt-ResourcePack.zip']
    },

Having an external reference to these assets wasn't the best idea, in retrospect, but I'm not sure of a good way to embed them in this library... (hardcode the zip in JS?)