Improve Emoji.parse typing

[beagold]

Summary

Found this while trying out the lib. Makes these situations nicer to handle:

emoji = hikari.Emoji.parse(emoji_str)

if isinstance(emoji, hikari.UnicodeEmoji):
    # Do something with UnicodeEmoji
else:
    # Do something with CustomEmoji
    #
    # Currently, the type of emoji here will be `Emoji`, which is quite unhelpful

Checklist

• I have run nox and all the pipelines have passed.
• I have made unittests according to the code I have added/modified/deleted.

Related issues

None