Convert commonjs code to esm during build (primer#2150)

* Build theme-preval to esm

* Use default export for theme-preval

* Create sweet-elephants-end.md

* Update theme-preval snapshot

.changeset/sweet-elephants-end.md

@@ -0,0 +1,5 @@
+---
+"@primer/react": patch
+---
+
+Ensure all files in `lib-esm` are in fact esm and not CommonJS

babel.config.js

@@ -22,7 +22,11 @@ module.exports = {
     development: {
       presets: makePresets(process.env.BABEL_MODULE || false),
       plugins: [
-        ...(process.env.BABEL_MODULE === 'commonjs' ? ['@babel/plugin-transform-modules-commonjs'] : []),
+        ...(process.env.BABEL_MODULE === 'commonjs'
+          ? ['@babel/plugin-transform-modules-commonjs']
+          : [
+              'transform-commonjs' // theme-preval is commonjs and needs to be transformed to esm
+            ]),
         ...sharedPlugins,
         replacementPlugin('development')
       ]

package.json

@@ -140,6 +140,7 @@
     "babel-plugin-open-source": "1.3.2",
     "babel-plugin-preval": "5.0.0",
     "babel-plugin-styled-components": "2.0.2",
+    "babel-plugin-transform-commonjs": "1.1.6",
     "babel-plugin-transform-replace-expressions": "0.2.0",
     "babel-polyfill": "6.26.0",
     "chroma-js": "2.1.2",