feat(Client): add typed wrapper methods for EventEmitter.{on,once}

[Renegade334]

So...

Historically, the discord.js typings have overloaded EventEmitter.on() and EventEmitter.once() with signatures that return typed tuples from ClientEvents, if the target is a client object and the event is in ClientEvents. This allowed users to, for example, do things like const [ role ] = await EventEmitter.once(client, 'roleCreate') and have the yielded values be automatically typed without needing any manual coercion.

These overloads had to be removed, due to TypeScript patching out the bug that was being exploited to add them to EventEmitter itself. They were moved downstream to Client.{on,once} in #10360 as a temporising measure, but the overload signatures themselves remained the same.

Unfortunately, EventEmitter.on() returns an async iterator.

Iterators are in a state of heavy flux within the TS library at the moment, and hence also within @types/node, which recently changed the types of its returned iterators across the entire library. These were formerly [Async]IterableIterators, but are now internal interfaces which extend TypeScript's [Async]IteratorObject in TS 5.6+.

Since those interfaces change shape depending on which TypeScript libs are loaded, this has the potential to cause subtle assignability errors with the Client overloads (eg. #10577) that are conditional on both TS and @types/node versions and also on which libs are loaded, and are therefore not picked up within the library's build tests. Even if the Client overloads are fixed now, they're going to be vulnerable to further breakage in the future if those interfaces continue to evolve.

However, there is a different approach entirely: declare new client instance methods, which wrap the EventEmitter static methods.

Instead of playing with EventEmitter definitions, wrapper methods are declared on the BaseClient prototype (naming up for discussion):

• .eventIterator(eventName[, options]) as a wrapper for EventEmitter.on(client, eventName[, options])
• .awaitEvent(eventName[, options]) as a wrapper for EventEmitter.once(client, eventName[, options])

This has a number of advantages:

• Offers users a stable API, any changes to which can respect discord.js semantic versioning.
• Offers a simpler call signature: client.awaitEvent(event) rather than EventEmitter.once(client, event).
• Makes the type definitions independent of users' TypeScript and @types/node versions (since they only depend on Promise and AsyncIterableIterator).
• Obviates the need to mess with the @types/node library definitions altogether, which removes the potential for further breakage in the future.

Removing the client-specific Client.{on,once} overload definitions from the .d.ts won't break any existing builds, since the only potential change from reverting to the native @types/node signatures is to widen the types of the yielded/resolved arrays (from typed tuples to any[]).

Resolves: #10577

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 changes the library's interface (methods or parameters added)

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