fix(polyfill): use core-js promise and iife fetch polyfill

[warapitiya]

Current polyfills are UMD supported. When appending Stencil.js to require.js web apps. IE breaks with an error Promise is undefined. This is due to es6-promise's UMD supported check.

typeof define === 'function' && define.amd ? define(factory) :

When I do a quick scan I found out that currently fetch polyfill also does a UMD check.

This PR will replace those 2 polyfills with below:

• Remove es6-promise polyfill and replace it with core-js/es/promise.
• Use fetch polyfill's IIFE version.

Issue related: #2418

[adamdbradley]

Thanks for the PR!