fix(@angular/cli): add $schema as a schema prop (angular#4779)

Fixes "Property $schema is not allowed" error on editors.

Author: filipesilva

packages/@angular/cli/lib/config/schema.json

@@ -4,6 +4,9 @@
   "title": "Angular CLI Config Schema",
   "type": "object",
   "properties": {
+    "$schema": {
+      "type": "string"
+    },
     "project": {
       "description": "The global configuration of the project.",
       "type": "object",