[breaking] Use Babel 8

[nicolo-ribaudo]

Follow up to #250

TODO(this PR):

• Test their compatibility with Babel 7 on CI

TODO(after this PR):

• 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