Prepare for Babel 8

[nicolo-ribaudo]

This PR aligns the repository with the way we are doing Babel 8:

• Removed babel-plugin-polyfill-corejs2
• Removed the CJS build
• Use jest-light-runner to run tests as actual CJS
• Updated the engines.node to match the other Babel packages
• Removed some now unnecessary dependencies

It does not actually update the packages to depend on Babel 8 stuff rather than Babel 7 stuff. The next steps here are:

• Update these packages to depend on Babel ^8.0.0-rc.2 <8.0.0
• (maybe) Test their compatibility with Babel 7 on CI
• Probably release them as 1.0.0-rc.1 (they have been stable for a while now)
• Before releasing Babel 8, update them to depend on 8.0.0-rc.<latest rc number> || ^8.0.0
• Release them as 1.0.0
• Update Babel 8 to depend on version ^1.0.0 of these packages
• Release Babel 8.0.0
• Update these packages to depend only on Babel ^8.0.0

If needed, we can keep the current Babel-7-compatible packages in a branch, just in case we'll need security fixes or critical backports.