dotnet-new csproj templates (#4382)

Make csproj templates first-class in dotnet-new.

TestAssets/TestProjects/ProjectJsonWebTemplate/Program.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Hosting;
+
+namespace WebApplication
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            var host = new WebHostBuilder()
+                .UseKestrel()
+                .UseContentRoot(Directory.GetCurrentDirectory())
+                .UseIISIntegration()
+                .UseStartup<Startup>()
+                .Build();
+
+            host.Run();
+        }
+    }
+}