improve structure & switch to yarn

babel.config.base.js

@@ -0,0 +1,28 @@
+module.exports = function (api) {
+  api.cache.using(() => process.env.NODE_ENV);
+
+  if (api.env('build')) {
+    return {
+      plugins: ['@babel/plugin-transform-runtime', '@babel/plugin-proposal-class-properties'],
+    };
+  }
+
+  if (api.env('test')) {
+    return {
+      plugins: ['@babel/plugin-transform-modules-commonjs'],
+      presets: [
+        [
+          '@babel/preset-env',
+          {
+            targets: {
+              node: 'current',
+            },
+          },
+        ],
+        '@babel/preset-typescript',
+      ],
+    };
+  }
+
+  return {};
+};