fix(deps): pin TypeScript below 3.7.0

Author: Benjamin E. Coe

handwritten/datastore/package.json

@@ -91,6 +91,6 @@
     "proxyquire": "^2.1.0",
     "sinon": "^7.3.2",
     "tmp": "0.1.0",
-    "typescript": "~3.7.0"
+    "typescript": "3.6.4"
   }
 }