extension guide and example (#623)

* extension guide and example

* code and guide fixes

* update dependencies

src/app/extensions.module.ts

@@ -1,6 +1,7 @@
 import { NgModule } from '@angular/core';
 import { AcaDevToolsModule } from 'aca-dev-tools';
 import { CodeEditorModule } from '@ngstack/code-editor';
+import { MyExtensionModule } from 'my-extension';
 
 // Main entry point for external extensions only.
 // For any application-specific code use CoreExtensionsModule instead.
@@ -13,7 +14,8 @@ import { CodeEditorModule } from '@ngstack/code-editor';
             // use local Typings Worker
             typingsWorkerUrl: 'assets/workers/typings-worker.js'
         }),
-        AcaDevToolsModule
+        AcaDevToolsModule,
+        MyExtensionModule
     ]
 })
 export class AppExtensionsModule {}