Replaced package.manifest with new BackOfficeJavaScriptAsset class

Author: abjerner

src/Skybrud.Umbraco.MultiNodeTreePicker/App_Plugins/Skybrud.Umbraco.MultiNodeTreePicker/package.manifest

@@ -1,13 +1,27 @@
 using Skybrud.Umbraco.MultiNodeTreePicker.Converters;
 using Umbraco.Cms.Core.Composing;
 using Umbraco.Cms.Core.DependencyInjection;
+using Umbraco.Cms.Core.WebAssets;
 
 namespace Skybrud.Umbraco.MultiNodeTreePicker.Composers {
 
     internal sealed class MntpComposer : IComposer {
 
         public void Compose(IUmbracoBuilder builder) {
             builder.WithCollectionBuilder<MntpConverterCollectionBuilder>().Add(() => builder.TypeLoader.GetTypes<IMntpItemConverter>());
+            builder.BackOfficeAssets().Append<BackOfficeJavaScriptAsset>();
+        }
+
+        public class BackOfficeJavaScriptAsset : IAssetFile {
+        
+            public AssetType DependencyType => AssetType.Javascript;
+
+            public string FilePath { get; set; }
+
+            public BackOfficeJavaScriptAsset() {
+                FilePath = "/App_Plugins/Skybrud.Umbraco.MultiNodeTreePicker/MntpConverter.js";
+            }
+
         }
 
     }