Include babel-polyfill in a Gutenberg 3.8 compatible way

[atimmer]

After WordPress/gutenberg#9643 in Gutenberg, our babel polyfill will throw an error that there shouldn't be two babel polyfills.

We should fix this by putting our babel polyfill in a separate file/bundle completely. Then we use this as a dependency of our scripts if Gutenberg isn't around. If Gutenberg is around we can use wp-polyfill-ecmascript as a script handle.

I've put this on 8.3, as this isn't released in Gutenberg yet. And: Everything keeps on working if babel-polyfill throws this error. It is only an error in the console.

[dariaknl]

re-tested fix in IE11 and Edge with the following setup:

• Gutenberg active, Gutenberg editor
• Gutenberg deactived, default classic editor
• Gutenberg active, Classic editor plugin ("replace Gutenberg editor" setting)
• Gutenberg active, Classic editor plugin ("use Gutenberg editor" setting)