feat: add Reaction and ChatReaction comms messages for emoji reactions

[biotech77]

Summary

Adds two new message types to the comms protocol (RFC-4) for emoji reactions:

• Reaction — Situational emoji reactions broadcast to nearby users (world-space floating emojis)
• ChatReaction — Reactions tied to a specific chat message, routed through the same pipe as the original message

Protocol Changes

Packet oneof

• Reaction reaction = 14
• ChatReaction chat_reaction = 15

Message definitions

Reaction (situational — nearby broadcast via island/scene pipes)

• emoji_index (int32) — index into the emoji atlas

ChatReaction (message-specific routed like chat messages)

• emoji_index (int32) index into the emoji atlas
• message_id (string) composite key (timestamp+wallet) identifying the target chat message
• address (string) sender wallet address,

Context

This is part of the chat reactions feature being implemented in the Explorer. Reactions allow users to send emoji
responses that appear as floating particles in world-space (situational) or attached to specific chat messages (chat
reactions).

[github-actions]

Test this pull request on NPM or Yarn

• The @dcl/protocol package can be tested in scenes by running the following NPM command:

  npm install "https://sdk-team-cdn.decentraland.org/@dcl/protocol/branch//dcl-protocol-1.0.0-22997207545.commit-32bf1fb.tgz"

• The @dcl/protocol package can be tested in scenes by running the following YARN command:

  yarn add "https://sdk-team-cdn.decentraland.org/@dcl/protocol/branch//dcl-protocol-1.0.0-22997207545.commit-32bf1fb.tgz"