Enable optional_chaining in open source Flow config

Summary:
There's a couple of Flow errors in open source, following D9081343:

```
yarn run v1.6.0
$ flow check
Error --------------------------------------------------------------------- Libraries/Components/Switch/Switch.js:139:31

Experimental optional chaining (`?.`) usage. Optional chaining is an active early-stage feature proposal that may
change. You may opt in to using it anyway by putting `esproposal.optional_chaining=enable` into the `[options]` section
of your `.flowconfig`.

   139|     let _trackColorForFalse = trackColor?.false;
                                      ^^^^^^^^^^^^^^^^^

Error --------------------------------------------------------------------- Libraries/Components/Switch/Switch.js:140:30

Experimental optional chaining (`?.`) usage. Optional chaining is an active early-stage feature proposal that may
change. You may opt in to using it anyway by putting `esproposal.optional_chaining=enable` into the `[options]` section
of your `.flowconfig`.

   140|     let _trackColorForTrue = trackColor?.true;
                                     ^^^^^^^^^^^^^^^^
```

See https://circleci.com/gh/facebook/react-native/48701

We have a different Flow config for open source, .github.flowconfig, that needs to be updated to ensure Flow passes in oss.

Reviewed By: TheSavior

Differential Revision: D9141134

fbshipit-source-id: 7b546ef4c9a91bdd66472242f38a6fc8bcf86364

.flowconfig

@@ -40,6 +40,8 @@ flow-github/
 [options]
 emoji=true
 
+esproposal.optional_chaining=enable
+
 module.system=haste
 module.system.haste.use_name_reducers=true
 # keep the following in sync with server/haste/hasteImpl.js