api.{Send,Edit}MessageData and their equivalents in package api/webhook
have been updated to add some fields added in Discord API v9.
(webhook.Client).EditMessage now also returns a message, because that
endpoint returns a message on success.
This commit moved ExecuteWebhookData from package api to package webhook
inside package api. This change required splitting the multipart
abstractions away from package api, so they are now inside package
sendpart in utils.
This commit will break code that uploads anything, as the type name is
now sendpart.File from api.SendMessageFile. The behavior should be the
same as before.
diamondburned