feat: Add support for Application Emoji

[silasary]

Pull Request Type

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

Description

New discord feature, applications can host emoji, rather than storing them in guilds.

Changes

• Added methods to Application for interacting with App Emoji
• Modifed PartialEmoji to allow for emoji without a guild_id
• Added missing available flag to PartialEmoji (This is technically unrelated, but I was in there)

Related Issues

Closes #1723

Test Scenarios

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