feat: add the on_disconnect event

[Catalyst4222]

About

This pull request creates the on_disconnect event
Closes #1180

Checklist

• The pre-commit code linter has been run over all edited files to ensure the code is linted.
• I've ensured the change(s) work on 3.8.6 and higher.

I've made this pull request: (check all that apply)

• For the documentation
• To add a new feature
• As a general enhancement
• As a refactor of the library/the library's code
• To fix an existing bug
• To resolve [REQUEST] Add event listener for bot disconnect #1180

This is:

• A breaking change

[i0bs]

cc @DeltaXWizard