Restore core-js and useBuiltIns = usage

[WraithKenny]

No description provided.

[gziolo]

See #9643 where useBuiltIns was disabled for production code.

This pull request seeks to disable our use of the @babel/runtime useBuiltIns option, replaced instead by @babel/polyfill as a dependency of every script registered by Gutenberg. This could be further enhanced to conditionally load the script only for browsers where expected features are not supported, but at this time it is not evident how such a condition would be formed.

While this technically adds a new script to be loaded, there is a net decrease in bundle size, from 2011.96kb total to 1803.72kb (+89.6kb polyfill) (minified, pre-gzip), for an overall savings of 118.64kb. Full build comparison below:

We include the following note for all packages that need polyfills (it will need to be updated when Babel 8 is shipped):

This package assumes that your code will run in an ES2015+ environment. If you're using an environment that has limited or no support for ES2015+ such as lower versions of IE then using core-js or @babel/polyfill will add support for these methods. Learn more about it in Babel docs.

What would be the reason to change it back to enable useBuiltIns?

[gziolo]

I'm closing this PR. Feel free to reopen if there is a compelling reason yo change the current approach.