dotnet-new csproj templates (#4382)

Make csproj templates first-class in dotnet-new.

TestAssets/TestProjects/ProjectJsonWebTemplate/Models/ApplicationUser.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
+
+namespace WebApplication.Models
+{
+    // Add profile data for application users by adding properties to the ApplicationUser class
+    public class ApplicationUser : IdentityUser
+    {
+    }
+}