types(GuildChannelManager): correct fetch return type

[RedGuy12]

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

The documentation for /channels/{channelId} says:

Get a channel by ID. Returns a channel object. If the channel is a thread, a thread member object is included in the returned result.

This causes me to believe that this route, and GuildChannelManager#fetch, can fetch threads. I tested it and it works.

Note that the bulk fetch type doesn't need to be updated, as the documentation for that specifically mentions the exclusion of threads:

Returns a list of guild channel objects. Does not include threads.

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

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
discord-js	✅ Ready (Inspect)	Visit Preview	Sep 11, 2022 at 6:27PM (UTC)

[Jiralite]

The documentation needs to be updated too:

discord.js/packages/discord.js/src/managers/GuildChannelManager.js

Lines 316 to 332 in 249ba0a

	/**
	* Obtains one or more guild channels from Discord, or the channel cache if they're already available.
	* @param {Snowflake} [id] The channel's id
	* @param {BaseFetchOptions} [options] Additional options for this fetch
	* @returns {Promise<?GuildChannel|Collection<Snowflake, GuildChannel>>}
	* @example
	* // Fetch all channels from the guild (excluding threads)
	* message.guild.channels.fetch()
	* .then(channels => console.log(`There are ${channels.size} channels.`))
	* .catch(console.error);
	* @example
	* // Fetch a single channel
	* message.guild.channels.fetch('222197033908436994')
	* .then(channel => console.log(`The channel name is: ${channel.name}`))
	* .catch(console.error);
	*/
	async fetch(id, { cache = true, force = false } = {}) {

GuildChannel doesn't include thread channels.

[RedGuy12]

The documentation needs to be updated too:

GuildChannel doesn't include thread channels.

Done.

[Jiralite]

Fetching multiple channels actually calls ChannelManager#_add() so it appears even channels in the collection may be null.

[codecov]

Codecov Report

Merging #8549 (4547153) into main (4ffdada) will increase coverage by 5.53%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #8549      +/-   ##
==========================================
+ Coverage   86.47%   92.01%   +5.53%     
==========================================
  Files          87       75      -12     
  Lines        8675     7160    -1515     
  Branches     1110     1023      -87     
==========================================
- Hits         7502     6588     -914     
+ Misses       1131      531     -600     
+ Partials       42       41       -1     

Flag	Coverage Δ
builders	100.00% <100.00%> (ø)
collection	100.00% <ø> (ø)
proxy	74.10% <ø> (ø)
rest	91.99% <ø> (ø)
utilities	100.00% <ø> (ø)
voice	63.86% <ø> (ø)
ws	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/collection/src/collection.ts	100.00% <ø> (ø)
packages/voice/src/DataStore.ts	98.14% <ø> (ø)
packages/voice/src/networking/Networking.ts	3.18% <ø> (ø)
packages/voice/src/networking/VoiceUDPSocket.ts	90.47% <ø> (ø)
packages/voice/src/networking/VoiceWebSocket.ts	0.00% <ø> (ø)
packages/voice/src/receive/SSRCMap.ts	86.20% <ø> (ø)
packages/voice/src/receive/SpeakingMap.ts	100.00% <ø> (ø)
packages/voice/src/util/Secretbox.ts	33.33% <ø> (ø)
...s/contextMenuCommands/ContextMenuCommandBuilder.ts	100.00% <100.00%> (ø)
.../interactions/slashCommands/SlashCommandBuilder.ts	100.00% <100.00%> (ø)
... and 12 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more