fix: load correct messaging module in browser-only

The frontend generator erroneously included the Electron messaging
module instead of the browser messaging module. Consequently, the
browser-only environment attempted to establish a WebSocket connection
to the backend. This issue has now been resolved.

Fixes #13820

dev-packages/application-manager/src/generator/frontend-generator.ts

@@ -108,7 +108,7 @@ ${Array.from(frontendPreloadModules.values(), jsModulePath => `\
 }
 
 module.exports = (async () => {
-    const { messagingFrontendModule } = require('@theia/core/lib/${this.pck.isBrowser()
+    const { messagingFrontendModule } = require('@theia/core/lib/${this.pck.isBrowser() || this.pck.isBrowserOnly()
                 ? 'browser/messaging/messaging-frontend-module'
                 : 'electron-browser/messaging/electron-messaging-frontend-module'}');
     const container = new Container();