Immortalizer

Adds "The Immortalizer" (and commands) for immortalizing mobs.

Immortalizer is directly inspired by a post I saw on Reddit by
u/LeadershipFlimsy9676
which requested a way to "immortalize mobs that normally can't be,"
I figured I could do something along those lines. So...

Features

• The Immortalizer: An item to target mobs you want to immortalize
  • Use on a mob to immortalize it, or attack it to make it mortal again.
• /immortalize <targets>: A command to target mobs you want to immortalize
  • Toggles immortality on the selected entity/entities

"The Immortalizer" Recipe

In an effort to give some viability to this item in a Survival playthrough,
I have made the recipe rather ridiculous. Feel free to change it with a datapack
or something like KubeJS/another approach.

{
  "type": "minecraft:crafting_shaped",
  "category": "misc",
  "key": {
    "H": "minecraft:dragon_head",
    "Q": "minecraft:quartz_block",
    "S": "minecraft:stick"
  },
  "pattern": [
    "H",
    "Q",
    "S"
  ],
  "result": {
    "count": 1,
    "id": "immortalize:immortalizer"
  }
}

/immortalize <targets> Command

It's pretty straight-forward; <targets> works just like
the entity selector for the /give
command in Vanilla Minecraft. Wiki: /give,
and targeting any LivingEntity will toggle it's tracked Immortality value.