Expose isAtom from flipper-plugin

Summary: Required for companion in later diffs

Reviewed By: passy

Differential Revision: D36517737

fbshipit-source-id: ad47ae00cc88f424507284553c7d4eb430691311

desktop/flipper-plugin/src/__tests__/api.node.tsx

@@ -57,6 +57,7 @@ test('Correct top level API exposed', () => {
       "createState",
       "createTablePlugin",
       "getFlipperLib",
+      "isAtom",
       "path",
       "produce",
       "renderReactRoot",

desktop/flipper-plugin/src/index.tsx

@@ -34,7 +34,7 @@ export {
   SandyPluginContext as _SandyPluginContext,
   usePlugin,
 } from './plugin/PluginContext';
-export {createState, useValue, Atom} from './state/atom';
+export {createState, useValue, Atom, isAtom} from './state/atom';
 export {batch} from './state/batch';
 export {
   FlipperLib,

docs/extending/flipper-plugin.mdx

@@ -1332,6 +1332,12 @@ someService.on('event', (val) => controllerPromise.resolve(val))
 await controllerPromise.promise
 ```
 
+### isAtom
+
+Usage: `isAtom(value)`
+
+Checks if value is an [Atom](https://github.com/facebook/flipper/blob/566f7108c9efd8aefc8522af645bd0674fb91b20/desktop/flipper-plugin/src/state/atom.tsx#L30). Returns boolean.
+
 ## TestUtils
 
 The object `TestUtils` as exposed from `flipper-plugin` exposes utilities to write unit tests for Sandy plugins.