feat(WebhookClient): allow creation of clients via URLs

[J-Human]

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

This allows WebhookClient's to be made with URLs, without the need of removing it or doing any type of parsing. This is useful for pass-through services like Webhook senders, etc., allowing a more convenient time for developers who don't want to use IDs and tokens. Example:

import { WebhookClient } from 'discord.js';

const webhook = new WebhookClient({ url: 'https://discord.com/api/webhooks/1234567890/abc' });

webhook.send({ content: 'Hi.' });

This PR also updates:

• Typings.
• Errors (URLs are passed to a RegEx and validated, although 100% perfection is not possible).
• Tests.

It now mandates initializing Webhook clients should have one object parameter, that can contain either ID + token or just a URL (this isn't validated so all 3 are optional).

Fixes #6164

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
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[J-Human]

@vladfrangu fixed! Thanks for reminding. 😅

[NotSugden]

the constructor typings need updating still

[J-Human]

@NotSugden @Jiralite done! Your suggestions have been committed. Thanks a lot.

• According to Jiralite's comment (and Crawl's decision) - the discordapp domain in the RegExp has vanished.
• As per Sugden's comment here, I updated the constructor typings for WebhookClient (yes, I forgot to do so), and retrieving the data from either the URL or ID, token has been refactored.

[J-Human]

Done.

[iCrawl]

Love it, I am actually in need of this too :D

[Alex-451]

Thanks for the addition @J-Human