Emoji reactions

Emoji reactions seem pretty essential for our use cases, so I'd like to propose a simple approach based on FEP-c0e0: https://fediverse.codeberg.page/fep/fep/c0e0/

Like with emoji in content for simple reactions:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams"
  ],
  "actor": "https://alice.social/users/alice",
  "content": "🔥",
  "id": "https://alice.social/activities/65379d47-b7aa-4ef6-8e4f-41149dda1d2c",
  "object": "https://bob.social/objects/57caeb99-424c-4692-b74f-0a6682050932",
  "to": [
    "https://alice.social/users/alice/followers",
    "https://bob.social/users/bob"
  ],
  "type": "Like"
}

EmojiReact activity when using custom emoji:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "toot": "http://joinmastodon.org/ns#",
      "Emoji": "toot:Emoji",
      "litepub": "http://litepub.social/ns#",
      "EmojiReact": "litepub:EmojiReact"
    }
  ],
  "actor": "https://alice.social/users/alice",
  "content": ":blobwtfnotlikethis:",
  "id": "https://alice.social/activities/65379d47-b7aa-4ef6-8e4f-41149dda1d2c",
  "object": "https://bob.social/objects/57caeb99-424c-4692-b74f-0a6682050932",
  "tag": [
    {
      "icon": {
        "mediaType": "image/png",
        "type": "Image",
        "url": "https://alice.social/files/1b0510f2-1fb4-43f5-a399-10053bbd8f0f"
      },
      "id": "https://alice.social/emojis/blobwtfnotlikethis",
      "name": ":blobwtfnotlikethis:",
      "type": "Emoji",
      "updated": "2024-02-07T02:21:46.497Z"
    }
  ],
  "to": [
    "https://alice.social/users/alice/followers",
    "https://bob.social/users/bob"
  ],
  "type": "EmojiReact"
}

I'd suggest adding these examples to the spec along with something along these lines:

Clients MAY send reactions using Like, optionally including a single Unicode emoji in content. Clients MAY send custom emoji reactions using EmojiReact.

Upon receiving a Like or EmojiReact, clients MUST indicate that the object has been reacted to, and SHOULD display the actors and aggregated counts per emoji.

If a Like includes an emoji in content, clients SHOULD render it as the reaction. For EmojiReact, clients SHOULD render the referenced emoji, or MAY fall back to content or a generic reaction if needed.

Clients MUST support undoing reactions via Undo, where the object is the original reaction activity ID (i.e., the encrypted reaction activity, not the transport wrapper).

Start by reading the existing specification and the linked FEP-c0e0, then compare its reaction guidance with the Like and EmojiReact examples in this issue. Done means the spec includes the proposed examples, reaction and aggregation requirements, custom-emoji fallback behavior, and Undo semantics for the original reaction activity.