feat: add new reaction event

[TyrantKingBen]

Pull Request Type

• Feature addition
• Bugfix
• Documentation update
• Code refactor
• Tests improvement
• CI/CD pipeline enhancement
• Other: [Replace with a description]

Description

Adds an event and a processor for when a message has all reactions of a specific emoji removed.

Changes

• Added MessageReactionRemoveEmoji event.
• Added MessageReactionRemoveEmoji processor (_on_raw_message_reaction_remove_emoji) to ReactionEvents event mixin.

Related Issues

Closes #1639.

Test Scenarios

Add some emoji reactions to a message. Right-click the message and remove all of a specific reaction under the "remove reactions" option. Right-click again and remove all reactions with the "remove all reactions" option. You should see that both events are being processed.

@listen(MessageReactionRemoveAll)
async def on_message_reaction_remove_all(event: MessageReactionRemoveAll) -> None:
    print(f"all emojis removed from message")

@listen(MessageReactionRemoveEmoji)
async def on_message_reaction_remove_emoji(event: MessageReactionRemoveEmoji) -> None:
    print(f"all {event.emoji} emojis removed from message")

Python Compatibility

• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x

Checklist

• I've run the pre-commit code linter over all edited files
• I've tested my changes on supported Python versions
• I've added tests for my code, if applicable
• I've updated / added documentation, where applicable