Add "display_entity" display settings to json models

This would allow you to set the default position an item appears in when a display entity is summoned (instead of just "none"), while also providing a display option that isn't used anywhere else in the game.

{
    "parent": "builtin/generated",
    "gui_light": "front",
    "display": {
        "ground": {
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 2, 0],
            "scale":[ 0.5, 0.5, 0.5 ]
        },
        "head": {
            "rotation": [ 0, 180, 0 ],
            "translation": [ 0, 13, 7],
            "scale":[ 1, 1, 1]
        },
        "thirdperson_righthand": {
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 3, 1 ],
            "scale": [ 0.55, 0.55, 0.55 ]
        },
        "firstperson_righthand": {
            "rotation": [ 0, -90, 25 ],
            "translation": [ 1.13, 3.2, 1.13],
            "scale": [ 0.68, 0.68, 0.68 ]
        },
        "fixed": {
            "rotation": [ 0, 180, 0 ],
            "scale": [ 1, 1, 1 ]
        },
        "display_entity": {
            "scale": [ 1, 1, 1 ]
      }
    }
}