provide empty mock for child_process so importing libraries with it works (#3033)

packages/react-scripts/config/webpack.config.dev.js

@@ -280,6 +280,7 @@ module.exports = {
     fs: 'empty',
     net: 'empty',
     tls: 'empty',
+    child_process: 'empty',
   },
   // Turn off performance hints during development because we don't do any
   // splitting or minification in interest of speed. These warnings become

packages/react-scripts/config/webpack.config.prod.js

@@ -358,5 +358,6 @@ module.exports = {
     fs: 'empty',
     net: 'empty',
     tls: 'empty',
+    child_process: 'empty',
   },
 };