Added css bundle api. Improved css bundling code

I made the code cleaner as requested @Masu-Baumgartner  :>

Moonlight.ApiServer/Implementations/Startup/CoreAssetStartup.cs

@@ -0,0 +1,24 @@
+using Moonlight.ApiServer.Interfaces.Startup;
+using Moonlight.ApiServer.Services;
+
+namespace Moonlight.ApiServer.Implementations.Startup;
+
+public class CoreAssetStartup : IAppStartup
+{
+    private readonly BundleService BundleService;
+
+    public CoreAssetStartup(BundleService bundleService)
+    {
+        BundleService = bundleService;
+    }
+
+    public Task BuildApp(IHostApplicationBuilder builder)
+    {
+        BundleService.BundleCss("css/core.min.css");
+        
+        return Task.CompletedTask;
+    }
+
+    public Task ConfigureApp(IApplicationBuilder app)
+     => Task.CompletedTask;
+}