Migrate constants

Summary: We can't `*`-export as the symbols are invisible but being explicit about this seems like a good improvement anyway.

Reviewed By: danielbuechele

Differential Revision: D16666637

fbshipit-source-id: 8f9a14ed41f4d8ad93b4bab8dd54f3b6c9e24824

src/index.js

@@ -21,7 +21,7 @@ export type {PluginClient, Props} from './plugin.tsx';
 export type {MetricType} from './utils/exportMetrics.js';
 export {default as Client} from './Client.tsx';
 export {clipboard} from 'electron';
-export * from './fb-stubs/constants.js';
+export {default as constants} from './fb-stubs/constants.tsx';
 export {connect} from 'react-redux';
 export {selectPlugin} from './reducers/connections.tsx';
 export {getPluginKey, getPersistedState} from './utils/pluginUtils.js';