feat: extended image download opts

[ItzDerock]

Fixes #166, #137, #128

Adds a resolveImageSrc callback with type:

discord-html-transcripts/src/downloader/images.ts

Lines 6 to 17 in cc95e47

	/**
	* Callback used to save an image attachment.
	* The returned string is the URL that will be used in the transcript.
	*
	* `undefined` indicates to use the original attachment URL.
	* `null` indicates to not include the attachment in the transcript.
	* `string` indicates to use the returned URL as the attachment URL (base64 or remote image).
	*/
	export type ResolveImageCallback = (
	attachment: APIAttachment,
	message: APIMessage
	) => Awaitable<string | null | undefined>;

Introduces a TranscriptImageDownloader builder:

Compression, in my testing, was able to turn a 3.3MB png to a 37KB WebP image with no noticeable quality change unless you really zoom in.

What needs to be done before merging:

• Figure out a way to document all of this
• more testing