Introduce "npx react-native codegen" command

Summary:
This diff introduces the `npx react-native codegen` command.
It runs the codegen for the `package.json` file in current working directory.

Changelog: [General][Added] - Introduce "npx react-native codegen" command.

Reviewed By: cipolleschi

Differential Revision: D51495465

packages/react-native/react-native.config.js

@@ -24,6 +24,13 @@ module.exports = {
     bundleCommand,
     ramBundleCommand,
     startCommand,
+    {
+      name: 'codegen',
+      func: () =>
+        require('./scripts/codegen/generate-artifacts-executor').execute(
+          process.cwd(),
+        ),
+    },
   ],
   platforms: {
     ios: {