Add typings for process.env (facebook#5557)

Author: brunolemos

Diff for: config/react-app.d.ts

@@ -2,6 +2,16 @@
 // Do not edit this file. It's replaced every time you launch a toolbox action.
 // If you need to add additional declarations, please do so in a new file.
 
+declare namespace NodeJS {
+  interface Process {
+    env: {
+      [key: string]: string | undefined;
+      NODE_ENV: 'development' | 'production' | 'test';
+      PUBLIC_URL: string;
+    };
+  }
+}
+
 declare module '*.bmp' {
   const src: string;
   export default src;