types: allow message cached props to be narrowed (#6838)

Co-authored-by: Almeida <almeidx@pm.me>
Co-authored-by: Antonio Román <kyradiscord@gmail.com>
Co-authored-by: Vlad Frangu <kingdgrizzle@gmail.com>

src/structures/Message.js

@@ -889,6 +889,14 @@ class Message extends Base {
     return equal;
   }
 
+  /**
+   * Whether this message is from a guild.
+   * @returns {boolean}
+   */
+  inGuild() {
+    return Boolean(this.guildId);
+  }
+
   /**
    * When concatenated with a string, this automatically concatenates the message's content instead of the object.
    * @returns {string}