fix: typings for 'WebhookMessageOptions'

[izexi]

Adds the missing username and avatarURL props and removes the replyTo prop from WebhookMessageOptions interface.

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

[csuvajit]

I think you're missing split, code, allowedMentions and files

[izexi]

I think you're missing split and code

They exist on the MessageOptions interface, so they're already being picked from there