types(DirectoryChannel): Ensure directory channels cannot contain user mentions when stringified

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
Directory channels when stringified cannot possibly be a user mention (which is typed through the inheritance chain):

discord.js/packages/discord.js/typings/index.d.ts

Line 2901 in 1f5d71e

export class DirectoryChannel extends BaseChannel {

discord.js/packages/discord.js/typings/index.d.ts

Lines 938 to 955 in 1f5d71e

	export abstract class BaseChannel extends Base {
	public constructor(client: Client<true>, data?: RawChannelData, immediatePatch?: boolean);
	public get createdAt(): Date | null;
	public get createdTimestamp(): number | null;
	public id: Snowflake;
	public flags: Readonly<ChannelFlagsBitField> | null;
	public get partial(): false;
	public type: ChannelType;
	public get url(): string;
	public delete(): Promise<this>;
	public fetch(force?: boolean): Promise<this>;
	public isThread(): this is AnyThreadChannel;
	public isTextBased(): this is TextBasedChannel;
	public isDMBased(): this is PartialGroupDMChannel | DMChannel | PartialDMChannel;
	public isVoiceBased(): this is VoiceBasedChannel;
	public isThreadOnly(): this is ThreadOnlyChannel;
	public toString(): ChannelMention | UserMention;
	}

Also added tests for the remaining channels.

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

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