fix(ThreadManager): match parent ID when fetching a single thread

[imnaiyar]

Please describe the changes this PR makes and why it should be merged: Currently as it stands, when fetching a single thread, discord.js simply returns ChannelManager#fetch, so ideally, one can fetch any channel of a guild, not just a thread in the channel of the manager, this is misleading as the docs state the return type to be a ThreadChannel (or equivalent). This pr adds in a check to match the parent id of the returned channel with the id of manager's channel and throws an error if it doesn't match. It also makes sense to have it this way, as one would expect to only get a thread of the channel the manager is of, similar to how fetchActive (and others) do.

I don't think it's breaking in any way as it doesn't change the return type or anything, just adds in a way to ensure it returns a thread of the parent channel.

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