[babel] remove @babel/polyfill, make core-js devDep

[spalger]

Summary

It feels like we shouldn't be requiring core-js in eui, and should instead rely on the application to setup a polyfill that modifies globals. To do this i've followed the pattern used for customizing the babel es module config and disabled the uiBuiltIns: usage config when building for production.

This means that consumers of EUI need to use a polyfill of some sort (preferably core-js@3), which is why I've labeled this a breaking change, but in reality every consumer of eui should already be doing this it just wasn't explicitly required before.

Checklist

• This was checked in mobile
• This was checked in IE11
• This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• This was checked against keyboard-only and screenreader scenarios
• This required updates to Framer X components

[chandlerprall]

One last change, otherwise everything looks good in the various cases I've put the this through.

[chandlerprall]

CI had an http timeout

jenkins test this

[chandlerprall]

Changes LGTM; ran build locally and verified core-js imports are removed as desired, and builds continue to run as expected.

[thompsongl]

CI looks like its hanging again, but let's hold merging this until EUI@11.3.0 is released regardless

[thompsongl]

CL in its right place