Add messaging & messaging-internal libraries (#15711)

libs/messaging-internal/src/subject-message.sender.ts

@@ -0,0 +1,17 @@
+import { Subject } from "rxjs";
+
+import { CommandDefinition, Message, MessageSender } from "@bitwarden/messaging";
+
+import { getCommand } from "./helpers";
+
+export class SubjectMessageSender implements MessageSender {
+  constructor(private readonly messagesSubject: Subject<Message<Record<string, unknown>>>) {}
+
+  send<T extends Record<string, unknown>>(
+    commandDefinition: string | CommandDefinition<T>,
+    payload: Record<string, unknown> | T = {},
+  ): void {
+    const command = getCommand(commandDefinition);
+    this.messagesSubject.next(Object.assign(payload ?? {}, { command: command }));
+  }
+}