feat(@schematics/angular): Add prompt for new apps to be zoneless

This commit adds a prompt to ask if the user wants to generate the app
as zoneless.

Author: atscott

packages/schematics/angular/application/schema.json

@@ -120,6 +120,7 @@
     },
     "zoneless": {
       "description": "Generate an application that does not use `zone.js`.",
+      "x-prompt": "Do you want to create the application without zone.js (zoneless, Developer Preview)?",
       "type": "boolean",
       "default": false
     }

packages/schematics/angular/ng-new/schema.json

@@ -141,6 +141,7 @@
     },
     "zoneless": {
       "description": "Create an initial application that does not utilize `zone.js`.",
+      "x-prompt": "Do you want to create the application without zone.js (zoneless, Developer Preview)?",
       "type": "boolean",
       "default": false
     }