Enforce spacing around * in generator functions (generator-star-spacing)

ESLint support the async keyword now, so we can re-enable this rule
that we previously disabled due to a bug:

Reverts:
18ab64fa9c623648
7caaba6ee3aa9e117eb94e5c

Fixes: standard/standard#724

eslintrc.json

@@ -39,6 +39,7 @@
     "eol-last": "error",
     "eqeqeq": ["error", "always", { "null": "ignore" }],
     "func-call-spacing": ["error", "never"],
+    "generator-star-spacing": ["error", { "before": true, "after": true }],
     "handle-callback-err": ["error", "^(err|error)$" ],
     "indent": ["error", 2, { "SwitchCase": 1 }],
     "key-spacing": ["error", { "beforeColon": false, "afterColon": true }],