feat: on_start event

[Toricane]

About

This pull request adds the on_start event.

Previous method of executing code when the bot is started:

started = False

@client.event
async def on_ready():
    global started
    if started:
        return
    started = True
    ...  # do stuff here

The PR implements this, which accomplishes the same thing:

async def on_start():
    ...  # do stuff here

Justification: most users do not check if the bot had started in their on_ready functions.
This would lead to it being dispatched multiple times, even though the majority wouldn't want it to.
Adding the on_start event mitigates this issue without breaking any functionality.

Checklist

• I've ran pre-commit to format and lint the change(s) made.
• I've checked to make sure the change(s) work on 3.8.6 and higher.
• This fixes/solves an Issue.
  • (If existent):
• I've made this pull request for/as: (check all that apply)
  • Documentation
  • Breaking change
  • New feature/enhancement
  • Bugfix