Add core-js support for build script

[fisker]

Since we've already drop Node 6 support for direct install from github, we could start using some new Syntax like async, Array.includes and more, at least for core bundles.

Also, some new version of dependencies requires async and Array#include, but our build script can't handle, like globby@10(#6776), cosmiconfig@6(#6850).

I add this as an option, and not enable for any bundle yet, also only allow to use for core bundles.

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• (If the change is user-facing) I’ve added my changes to the CHANGELOG.unreleased.md file following the template.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[lydell]

As far as I understand, we won’t need this in 2.0 so I’m closing. Let me know if I’ve misunderstood anything.

[lydell]

@fisker Feel free to continue discussion here. We’ll reopen if needed.

[fisker]

@lydell You are right, we can reopen it when we need it