custom_model_data should be key:value pairs instead of indexed lists

Currently with the new changes, the custom_model_data is changed into 4 different lists of floats, flags, strings, and colors.  A more useful implementation would be to have key: value pairs instead.

The issue with this implementation is that it immediately creates problems when trying to add, remove, or modify data.  For example, if you were trying to add a custom transmute recipe to modify a model in some way, it would replace the entire custom model data list instead of adding or removing a single index value.  Another issue is that it creates a large number of compatibility issues with datapacks, as multiple packs may try adding or removing values to these indexed lists without any proper way of knowing which index is used for the corresponding datapack.

The better solution would be to have `key: value` pairs to accomplish this.  something like this:

{
  floats: {
    foo: 1.0,
    bar: 2.5
  },
  strings: {
    something: "this",
    second: "string"
  }
}