See #17146.

The regenerator repository has been archived and it's not maintained anymore (https://github.com/facebook/regenerator/). I suggest that we:

• Move regenerator-runtime into @babel/helpers. We already have a bundle step that bundles it into @babel/helpers, we can just make that the original source now.
• Move regenerator-transform into @babel/plugin-transform-generator, which will also simplify interaction with @babel/helpers.