chore(babel): use api.cache (webpack#2157)

Author: hiroppy

.babelrc

@@ -0,0 +1,14 @@
+'use strict';
+
+module.exports = (api) => {
+  api.cache(true);
+
+  return {
+    presets: ['@babel/preset-env'],
+    env: {
+      test: {
+        plugins: ['@babel/plugin-transform-runtime'],
+      },
+    },
+  };
+};