types: replace Mixins with interface merging

[Qjuh]

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

Replaces (Partial)TextBasedChannelMixin and (Partial)WebhookMixin with interface merging instead for better documentation while effectively offering the same types.

The DiscordjsErrorMixin isn't directly replaceable like this, because it narrows the name and code properties.

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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Name	Status	Preview	Comments	Updated (UTC)
discord-js	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 11, 2024 7:24pm
discord-js-guide	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 11, 2024 7:24pm

[Renegade334]

While in the vicinity, might be worth changing the following to match the implementation:

  /** @internal */
  export interface DiscordjsErrorFields<Name extends string> {
-   readonly name: `${Name} [${DiscordjsErrorCodes}]`;
-   get code(): DiscordjsErrorCodes;
+   get name(): `${Name} [${DiscordjsErrorCodes}]`;
+   readonly code: DiscordjsErrorCodes;
  }

[Qjuh]

Feel free to make a PR. I don't like to mix unrelated stuff into this one.