types(Partial): add partial reactions (#6066)

Co-authored-by: Noel <buechler.noel@outlook.com>

src/structures/Channel.js

@@ -63,6 +63,16 @@ class Channel extends Base {
     return new Date(this.createdTimestamp);
   }
 
+  /**
+   * Whether this Channel is a partial
+   * <info>This is always false outside of DM channels.</info>
+   * @type {boolean}
+   * @readonly
+   */
+  get partial() {
+    return false;
+  }
+
   /**
    * When concatenated with a string, this automatically returns the channel's mention instead of the Channel object.
    * @returns {string}