eslint: Enable no-var rule

`var` has broken/unexpected scoping in JavaScript, and can lead to
subtle errors. It's preferable and safer to use `const` whenever
possible, and `let` where necessary.

There are no usages of `var` in starter-kit right now, so just enable
the rule to ensure that it stays that way.

.eslintrc.json

@@ -23,6 +23,7 @@
                 "ignoredNodes": [ "JSXAttribute" ]
             }],
         "newline-per-chained-call": ["error", { "ignoreChainWithDepth": 2 }],
+        "no-var": "error",
         "lines-between-class-members": ["error", "always", { "exceptAfterSingleLine": true }],
         "prefer-promise-reject-errors": ["error", { "allowEmptyReject": true }],
         "react/jsx-indent": ["error", 4],