dillon1000/react

[playground] add flow parsing

I don't think there is a monaco config to parse flow (for IDE integration), but 
now we should be able to parse flow for the Forget compiler. 

Since typescript and flow has different parsing rules, playground will only use 
flow parser if `// @flow` is at the top of the file. 

Test: 
[link](https://react-forget-playground-j7nd7j8cv-fbopensource.vercel.app/#eyJzb3VyY2UiOiIvLyBAZmxvd1xuZnVuY3Rpb24gQ29tcG9uZW50KHByb3BzKSB7XG4gIGxldCB4ID0gKHByb3BzOiBudW1iZXIpO1xuICByZXR1cm4geDtcbn1cbiJ9)