Utility to determine if client is PWA

Summary: This is currently used in multiple places, let's put it under utils.

Reviewed By: aigoncharov

Differential Revision: D50883125

fbshipit-source-id: ee161a035cd01ca73a2521f813c4b397aa647037

desktop/flipper-ui-core/src/index.tsx

@@ -12,3 +12,5 @@ export {RenderHost, getRenderHostInstance} from 'flipper-frontend-core';
 export {startFlipperDesktop} from './startFlipperDesktop';
 
 export {Icon} from './utils/icons';
+
+export {isPWA} from './utils/pwa';