add INgModelController#$setPristine()

This exists in the 1.2.x API:

https://code.angularjs.org/1.2.27/docs/api/ng/type/ngModel.NgModelController

angularjs/legacy/angular-1.2.d.ts

@@ -285,6 +285,7 @@ declare module ng {
         // Documentation states viewValue and modelValue to be a string but other
         // types do work and it's common to use them.
         $setViewValue(value: any): void;
+        $setPristine(): void;
         $viewValue: any;
 
         $modelValue: any;