[Bug] Webhook issues

[Oliver-makes-code]

What's the bug?
Currently, webhooks have a couple problems:

1. Files are unable to be sent via webhooks.
2. The Webhook constructor accesses this.client before it's assigned. Fixed in PR Fix latter 2 issues of #273 #274
3. There's no proper way to fetch a Webhook from a channel without making direct API requests. Fixed in PR Fix latter 2 issues of #273 #274

With 2, it can be as easy as assigning this.client at the beginning of the constructior, with 3, a TextChannel$fetchWebhooks method could be added, and 1 has a more in-depth solution that I haven't figured out yet.

[DjDeveloperr]

Files can already be sent, just that sending multiple files is not supported at the moment (I will make PR for it). Take following example:

const hook = await Webhook.fromURL(Deno.env.get('WEBHOOK')!)
await hook.send({
  file: await MessageAttachment.load(
    'https://cdn.discordapp.com/emojis/783639203153575948.png?size=96'
  )
})