Export node as JSON

Summary:
There has been multiple requests to incorporate an export to plain text functionality for a while.

This diff adds it.

It will export a node and optionally its chidren as JSON.

Reviewed By: antonk52

Differential Revision: D49596476

fbshipit-source-id: 3681bc0c2d02e1ea64ff589e0e272f6d54ad0524

desktop/flipper-server-core/src/server/startServer.tsx

@@ -178,7 +178,9 @@ async function startHTTPServer(
   });
 
   return new Promise((resolve) => {
-    console.log(`Starting server on http://localhost:${config.port}`);
+    console.info(
+      `[flipper-server] Starting server on http://localhost:${config.port}`,
+    );
     const readyForIncomingConnections = (
       serverImpl: FlipperServerImpl,
       companionEnv: FlipperServerCompanionEnv,