Upgrade Babel to 7.0.0-rc.1 (major)

[kachkaev]

Fixes #4691
Fixes #4614

This PR gives path to vercel/next-plugins#242

I did not add or remove ^ near dependency versions in package.json files. However, I don't exclude that some changes can be made given that rc is more stable than beta.

[bensalilijames]

This should fix #4614 too! 🙌

[kachkaev]

@timneutkens 7.0.0-rc.2 is out and I will be happy to submit a new PR to this repo and to next-plugins. Do you pre-approve? I'm also wondering if we can simply specify ^7.0.0-rc.2 for all babel stuff given that RCs are usually more stable than beta. This will remove a need to bump next canary every time a new RC is out. WDYT?

If anything really bad happens in any of the RCs, canary testers will be able to manually freeze package versions in their package.json

[brainkim]

You can do “^7.0.0-r” if you want install the higest release candidate version

[timneutkens]

From earlier tweets babel 7 will be out this week

[kachkaev]

👍 great, let's wait!

[schoenwaldnils]

Babel 7.0.0 is out 🎉

[kachkaev]

@timneutkens will you be happy to accept a simple PR? Does anything need changing apart from the version bump?

[kachkaev]

Ah, it's already done! #5042 😅 👍