Tweak and re-organize ESLint config (#38369)

* Tweak and re-organize ESLint config

* merge individual configs to the root config
* enable more eslint-plugin-import rules
* lint markdown files

* Lint

js/tests/browsers.js

@@ -1,6 +1,7 @@
-/* eslint-env node */
 /* eslint-disable camelcase */
 
+'use strict'
+
 const browsers = {
   safariMac: {
     base: 'BrowserStack',