fix(babel-preset-gatsby): Add corejs to config

[erikdstock]

Description

Gatsby started warning in development mode about a missing but assumed corejs config that was subject to change. The issue linked below tracks down the source of the problem and this fix quiets it down. Thanks!

Related Issues

Fixes #12744

[erikdstock]

Hmmm. This is a bummer.

Several failed builds about putting corejs as a top-level option, but it's in the babel-preset-env source as of 3 days ago. It looks like this would be a version bump from 7.0.0 - 7.4.1 - happy to give it a go.

Summary of all failing tests
 FAIL  packages/gatsby/cache-dir/__tests__/minimal-config.js
  ● Builds cache-dir with minimal config

    expect(received).toEqual(expected)

    Difference:

    - Expected
    + Received

    + Error: [BABEL] /home/circleci/project/packages/gatsby/cache-dir/api-runner-browser-plugins.js: Invalid Option: corejs is not a valid top-level option.
    +         Maybe you meant to use 'targets'? (While processing: "/home/circleci/project/node_modules/@babel/preset-env/lib/index.js")
    +     at validateTopLevelOptions (/home/circleci/project/node_modules/@babel/preset-env/lib/normalize-options.js:49:13)

[KyleAMathews]

From my read of https://babeljs.io/blog/2019/03/19/7.4.0#migration-from-core-js-2 — we need to use core-js@3 not 2 because we're using @babel/preset-env.

[erikdstock]

Thanks @KyleAMathews - I tried to do it the fast way with a find/replace on package.jsons and although tests passed it's not running in my test app. The link you mentioned above actually says that core-js@2 will remain the default since 3 has breaking changes, which tracks with the warning message on build- saying that babel-preset-env is assuming 2, but that will change, so best to add the property to the config (this PR).

How would you feel about merging to appease babel for now and making followup issues to upgrade core-js in the gatsby and gastby-cli packages?

[KyleAMathews]

Ok, the blog post is a bit confusing then. Yeah, then going with 2 for now makes sense and we can upgrade when we release Gatsby v3.

[gatsbot]

Holy buckets, @erikdstock — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. (Currently we’ve got a couple t-shirts available, plus some socks that are really razzing our berries right now.)
2. We just invited you to join the Gatsby organization on GitHub. This will add you to our team of maintainers. Accept the invite by visiting https://github.com/orgs/gatsbyjs/invitation. By joining the team, you’ll be able to label issues, review pull requests, and merge approved pull requests.

If there’s anything we can do to help, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again!