Remove all usages of babel-node

[BPScott]

WHY are these changes introduced?

That extra transpile step means an extra dependency but doesn't add
any value

WHAT is this pull request doing?

• Replaces all calls to babel-node with node.
• Removes the dependency on babel-cli
• Updates all scripts so they work with plain node. This boiled down to replacing import lines with require() and replacing export default with module.exports =

How to 🎩

build and build-consumer

• Check out master branch
• Run mkdir -p old-build/node_modules/@shopify/polaris-react && yarn run build-consumer polaris-react/old-build This will create a baseline build to compare against
• Checkout this branch and yarn install
• Run yarn clean && mkdir -p new-build/node_modules/@shopify/polaris-react && yarn run build-consumer polaris-react/new-build This creates a new build
• Run diff -r -u old-build new-build to compare the build outputs. The only difference should be within the package.json

readme-update-version

• Modify the version in package.json (e.g change it to 3.4.0-rc.1)
• Run yarn run readme-update-version and see that README file contents have been updated

open-styleguide-pr

Kinda hard without making a release to test.

[tmlayton]

I trust that you tested this per the 🎩 instructions, otherwise code looks good. We have tests for the build scripts and the readme script, too.

Could you add a changelog entry just as a safety measure to provide more visibility around this change?

[BPScott]

Rebased to include a changelog entry 👍