Fixed the sinon global shim

Typescript still wasn't happy with the way we had it, but this way seems to work.

Author: KorinaGraber

typings/sinon.d.ts

@@ -1 +1,5 @@
-declare var sinon: Sinon.SinonStatic;
+import { SinonStatic } from 'sinon';
+
+declare global {
+  const sinon: SinonStatic;
+}