Add a plugin for detecting when the client joins or leaves a guild

[abitofevrything]

Description

Allows users to easily know whether a GuildCreateEvent originated from the client joining a guild (as opposed to it becoming available or during initial cache population) and whether a GuildDeleteEvent originated from the client leaving a guild (as opposed to a guild becoming unavailable due to an outage).

Type of change

• New feature (non-breaking change which adds functionality)

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