fix(Types): make event listeners accept async callbacks

[kyranet]

Please describe the changes this PR makes and why it should be merged:

When using client.on with a callback that returns a Promise, eslint will then complain if the no-misused-promises (from @typescript-eslint and various plugins) is enabled, as Promise<?> does not match void.

By making the return type of the callbacks unknown, we can now return any data, including promises.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR only includes non-code changes, like changes to documentation, README, etc.

[vladfrangu]

I dunno... the type would be void | Promise<void> at best... I'm personally fine with this, but I don't know what the rest of the team thinks