refactor: reintroduce validateThing method

src/common.ts

@@ -182,6 +182,20 @@ export function isSchemaWithDefinitions(schema: JSONSchema): schema is SchemaWit
   return typeof schema.definitions !== 'undefined';
 }
 
+// tslint:disable: completed-docs
+/**
+ * Guard method for determining if an object (a thing) has a type property with a type of string
+ *
+ * @param thing An object (thing)
+ */
+export function isThingWithType(thing: unknown): thing is { type: string; } {
+  return typeof thing === 'object' &&
+         thing !== null &&
+         'type' in thing &&
+         typeof (thing as { type: string; }).type === 'string';
+}
+// tslint:enable: completed-docs
+
 /**
  * Get path that contains a tsconfig.json
  *