Make eslint rule an error instead of a warning

This will `@typescript-eslint/no-unused-vars` an error instead of a warning. It's helpful for the rule to error so that the build will fail so that you don't have to check the logs to see if there was a warning. See mrdoob#20467 for context.

package.json

@@ -34,7 +34,7 @@
       "@typescript-eslint"
     ],
     "rules": {
-      "@typescript-eslint/no-unused-vars": 1,
+      "@typescript-eslint/no-unused-vars": "error",
       "@typescript-eslint/indent": [
         "error",
         "tab",