Add linter for JS files with basic recommended rules:

- Add Github Action.
- Fix JS files to match rules; mostly `globals` and `exported`.

.eslintrc.js

@@ -0,0 +1,10 @@
+/* global module */
+
+module.exports = {
+    "env": {
+        "browser": true,
+        "es6": true
+    },
+
+    "extends": "eslint:recommended"
+};