Extend Atom's update interface

Summary:
This help handle updates to objects containing JSONObject type (generates typescript errors around infinite recursion:

> Type instantiation is excessively deep and possibly infinite.ts(2589)

Reviewed By: mweststrate

Differential Revision: D28098502

fbshipit-source-id: a8e7f785b03bce7c8e1c0b355e2eca2e087b1881

desktop/flipper-plugin/src/state/atom.tsx

@@ -22,6 +22,7 @@ export interface ReadOnlyAtom<T> {
 export interface Atom<T> extends ReadOnlyAtom<T> {
   set(newValue: T): void;
   update(recipe: (draft: Draft<T>) => void): void;
+  update<X extends T>(recipe: (draft: X) => void): void;
 }
 
 class AtomValue<T> implements Atom<T>, Persistable {