Fix: display spellcasting information in the "Attacks & Spellcasting" box

[rg-wood]

Currently spellcasting is displayed in the "Features & Traits" section but more properly belongs in the "Attacks & Spellcasting" box.

Possible structure for spellcasting information:

{
  "spellcasting": {
    "ability": "charisma",
    "spells": {
      "cantrips": {
        "spells": ["light", "sacred flame"]
      },
      "1": {
        "slots": 2,
        "spells": ["Charm Person"]
      }
    }
  }
}

What to do when you have more than one kind of spellcasting, for example, multiclassing?