Add __ctx to Messages for optional comments. Move ServerMessage.ts up

ServerMessage.ts

@@ -4,6 +4,7 @@ export enum MessageType {
 }
 
 export type ServerMessage = {
+  __ctx?: string;
   type: MessageType;
   date: number;
 };
@@ -31,6 +32,13 @@ export function isServerMessage(
   ) {
     return false;
   }
+  if (
+    Object.hasOwnProperty.call(obj, "__ctx") &&
+    typeof (obj as { __ctx: unknown }).__ctx !== "string" &&
+    typeof (obj as { __ctx: unknown }).__ctx !== "undefined"
+  ) {
+    return false;
+  }
 
   return true;
 }