feat(core): CHECKOUT-3079 Enable no-used-variable rule

BREAKING CHANGE: You must now remove all unused variables.

Author: davidchin

index.json

@@ -115,6 +115,7 @@
         "no-unnecessary-initializer": true,
         "no-unsafe-finally": true,
         "no-unused-expression": true,
+        "no-unused-variable": true,
         "no-var-keyword": true,
         "no-var-requires": true,
         "object-literal-key-quotes": [