Replace all usages of var with let / const - eslint auto applied (#3444)

Author: brianc

.eslintrc

@@ -16,7 +16,9 @@
     "@typescript-eslint/no-unused-vars": ["error", {
       "args": "none"
     }],
-    "no-unused-vars": "off"
+    "no-unused-vars": "off",
+    "no-var": "error",
+    "prefer-const": "error"
   },
   "overrides": [
     {