build: use + for ES6 template literals

The behavior before is `"".concat(part1, part2, ...)`,
which is useless and makes the minified file larger (about 600 bytes).

The benefit of `concat` is just supporting `Symbol.toPrimitive`.
Therefore, we should replace it with simpler `+`.

Doc:
* https://babeljs.io/docs/en/babel-plugin-transform-template-literals
* babel/babel#5791

.babelrc

@@ -7,6 +7,11 @@
       }
     ]
   ],
+  "plugins": [
+    ["@babel/plugin-transform-template-literals", {
+      "loose": true
+    }]
+  ],
   "env": {
     "test": {
       "plugins": [