feat(Message): replace referencedMessage with fetchReference

[monbrey]

This replaces the cache-reliant Message#referencedMessage with a more reliable way to resolve the Message
This follows precedent set by #5480

The method with throw an error if Message#reference is null, or if the channel to fetch the message from is not in the client's cache, as is likely to be the case when the message is a crosspost rather than a reply.

This seemed like the best way to handle it without getting into raw APIs calls and partial channels and all that mess.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• I know how to update typings and have done so, or typings don't need updating

stale