Enable Flow casting_syntax=both in fbsource

Summary:
Enable Flow `casting_syntax=both` in fbsource, before the announcement so that when people see it they can check it out without rebasing.

bypass-github-export-checks
bypass-lint

Changelog: [Internal]

Reviewed By: SamChou19815

Differential Revision: D51097870

fbshipit-source-id: dfcb04000df26140c971422389b6fce0b1ba51e7

.flowconfig

@@ -33,6 +33,7 @@ packages/react-native/flow/
 [options]
 experimental.global_find_ref=true
 enums=true
+casting_syntax=both
 
 emoji=true