Use more precise emoji type in reaction events

[abitofevrything]

Description

Reaction events used to use PartialEmoji when the API returns an object that is always parseable to an Emoji. This PR changes these events to use the more precise type.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my changes haven't lowered code coverage