Update RCL template to use 5.0 features (#25613)

* Update JS interop to use auto-loaded ES6 module

* Test update

* Make it lazy for compatibility with prerendering

* Update comment

* Code style: go back to .AsTask - it's probably easier to read

Author: SteveSandersonMS

src/ProjectTemplates/Web.ProjectTemplates/content/RazorClassLibrary-CSharp/ExampleJsInterop.cs

@@ -1,16 +1,39 @@
-using Microsoft.JSInterop;
+using System;
 using System.Threading.Tasks;
+using Microsoft.JSInterop;
 
 namespace Company.RazorClassLibrary1
 {
-    public class ExampleJsInterop
+    // This class provides an example of how JavaScript functionality can be wrapped
+    // in a .NET class for easy consumption. The associated JavaScript module is
+    // loaded on demand when first needed.
+    //
+    // This class can be registered as scoped DI service and then injected into Blazor
+    // components for use.
+
+    public class ExampleJsInterop : IAsyncDisposable
     {
-        public static ValueTask<string> Prompt(IJSRuntime jsRuntime, string message)
+        private readonly Lazy<Task<IJSObjectReference>> moduleTask;
+
+        public ExampleJsInterop(IJSRuntime jsRuntime)
+        {
+            moduleTask = new (() => jsRuntime.InvokeAsync<IJSObjectReference>(
+                "import", "./_content/Company.RazorClassLibrary1/exampleJsInterop.js").AsTask());
+        }
+
+        public async ValueTask<string> Prompt(string message)
+        {
+            var module = await moduleTask.Value;
+            return await module.InvokeAsync<string>("showPrompt", message);
+        }
+
+        public async ValueTask DisposeAsync()
         {
-            // Implemented in exampleJsInterop.js
-            return jsRuntime.InvokeAsync<string>(
-                "exampleJsFunctions.showPrompt",
-                message);
+            if (moduleTask.IsValueCreated)
+            {
+                var module = await moduleTask.Value;
+                await module.DisposeAsync();
+            }
         }
     }
 }

src/ProjectTemplates/Web.ProjectTemplates/content/RazorClassLibrary-CSharp/wwwroot/exampleJsInterop.js

@@ -1,8 +1,6 @@
-// This file is to show how a library package may provide JavaScript interop features
-// wrapped in a .NET API
+// This is a JavaScript module that is loaded on demand. It can export any number of
+// functions, and may import other JavaScript modules if required.
 
-window.exampleJsFunctions = {
-  showPrompt: function (message) {
-    return prompt(message, 'Type anything here');
-  }
-};
+export function showPrompt(message) {
+  return prompt(message, 'Type anything here');
+}

src/ProjectTemplates/test/template-baselines.json

@@ -877,9 +877,9 @@
       "Files": [
         "wwwroot/background.png",
         "wwwroot/exampleJsInterop.js",
-        "wwwroot/styles.css",
         "_Imports.razor",
         "Component1.razor",
+        "Component1.razor.css",
         "ExampleJsInterop.cs"
       ]
     },