refactor: rewrite message creation (#2774)

* Rework createMessage
- MessageAttachment is now structurally similar to FileOptions
- No longer mutates the object passed as options
- Supports more permutations of arguments

* Ignore complexity warning

* Refactor name finding

* Fix typo

* Update typings

* Default name to null for MessageAttachment

* Make Message#reply use transformOptions

* Move transformOptions

* Fix Message#reply

* Fix mutation

* Update tests

* Fix options passing

* Refactor into APIMessage

* Fix webhook send

* Expose APIMessage

* Add documentation

* Add types

* Fix type doc

* Fix another type doc

* Fix another another type doc (is this one even right!?)

* Remove trailing comma

* Properly clone split options

* Add support for sending file as stream

* Missed a doc

* Resolve files only once when splitting messages

* This looks nicer

* Assign directly

* Don't cache data and files

* Missing return type

* Use object spread instead Object.assign

* Document constructors

* Crawl is a little dot

* comp pls

* tests: sanitize local file path, disable no-await-in-loop

src/structures/MessageEmbed.js

@@ -1,4 +1,3 @@
-const MessageAttachment = require('./MessageAttachment');
 const Util = require('../util/Util');
 const { RangeError } = require('../errors');
 
@@ -141,14 +140,8 @@ class MessageEmbed {
      * @type {Array<FileOptions|string|MessageAttachment>}
      */
     this.files = [];
-
     if (data.files) {
-      this.files = data.files.map(file => {
-        if (file instanceof MessageAttachment) {
-          return typeof file.file === 'string' ? file.file : Util.cloneObject(file.file);
-        }
-        return file;
-      });
+      this.files = data.files;
     }
   }
 
@@ -203,7 +196,6 @@ class MessageEmbed {
    * @returns {MessageEmbed}
    */
   attachFiles(files) {
-    files = files.map(file => file instanceof MessageAttachment ? file.file : file);
     this.files = this.files.concat(files);
     return this;
   }