Promote core-js/stable instead of @babel/polyfill

https://babeljs.io/docs/en/ says “Babel is a JavaScript compiler: Polyfill features that are missing in your target environment (through @babel/polyfill)”

https://babeljs.io/docs/en/usage says “Running these commands to install the packages: npm install --save @babel/polyfill… Polyfill: As of Babel 7.4.0, this package has been deprecated in favor of directly including core-js/stable (to polyfill ECMAScript features) and regenerator-runtime/runtime (needed to use transpiled generator functions)”

https://babeljs.io/docs/en/babel-polyfill repeats “As of Babel 7.4.0, this package has been deprecated in favor of directly including core-js/stable (to polyfill ECMAScript features) and regenerator-runtime/runtime (needed to use transpiled generator functions)”

• Change https://babeljs.io/docs/en/ to state “… (through core-js/stable)”

• Update the documentation to state first how to poliyfill with core-js and then how for old Babel to use @babel/polyfill.