fix(MessageMentions#channels): Fix type of channels of mentions

[GrapeColor]

Please describe the changes this PR makes and why it should be merged:
Change type of channels mentions from TextChannel to GuildChannel.
MessageMentions#channels are defined as Collection<Snowflake, TextChannels> even though they return Collection<Snowflake, GuildChannels>.

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

[almostSouji]

Incorrect, since we parse channel mentions from content and resolve them based on the available cache it can be any channel type, including DMChannel, the only requirement is <#id> to be present and the corresponding channel to be cached.

(meaning the docs and typings are wrong)

[GrapeColor]

When I verified it, it was exactly as you said.
I fixed it again and committed.

[almostSouji]

Docstring also needs updating

[GrapeColor]

Oops.
Excuse me again and again.

Is it all right like this?

[SpaceEEC]

Can you also update the types for the private properties?

discord.js/typings/index.d.ts

Line 1111 in 9581918

private _channels: Collection<Snowflake, GuildChannel> | null;

discord.js/src/structures/MessageMentions.js

Line 90 in 9581918

* @type {?Collection<Snowflake, GuildChannel>}

[GrapeColor]

Thank you for telling me.
It seems that #has() was also wrong, so I fixed it together.