Bump to Babel 8 once `@wordpress/babel-preset-default` and `@wordpress/eslint-plugin` are ready

[Hug0-Drelon]

@wordpress/babel-preset-default and @wordpress/eslint-plugin still require Babel 7, so Dependabot major bumps for @babel/cli / @babel/core break build and lint (#40).

We ignore those major updates for now (#42).

When upstream supports Babel 8:

• Remove the ignore rules in .github/dependabot.yml
• Bump Babel + WordPress packages together
• Verify npm ci, npm run build, npm run lint