DEV: Import MessageBus from message-bus-client instead of globals (#9902)

app/assets/javascripts/discourse-shims.js

@@ -0,0 +1,18 @@
+// Allow us to import Ember
+define("ember", ["exports"], function(__exports__) {
+  // eslint-disable-next-line no-console
+  console.warn(
+    [
+      "Deprecation notice:",
+      "Use specific `@ember/*` imports instead of `ember`",
+      "(deprecated since Discourse 2.4.0)",
+      "(removal in Discourse 2.5.0)"
+    ].join(" ")
+  );
+
+  __exports__.default = Ember;
+});
+
+define("message-bus-client", [], function() {
+  return window.MessageBus;
+});