Update to react-native 0.64 and enable Hermes

Summary:
allow-large-files

Per title. hermes enabled so that we can verify the hermes debugger works.

Reviewed By: passy

Differential Revision: D27229331

fbshipit-source-id: c2e45151b9d2097edc7e79f54eb89f9d110899d6

react-native/ReactNativeFlipperExample/.flowconfig

@@ -30,6 +30,8 @@ emoji=true
 esproposal.optional_chaining=enable
 esproposal.nullish_coalescing=enable
 
+exact_by_default=true
+
 module.file_ext=.js
 module.file_ext=.json
 module.file_ext=.ios.js
@@ -71,4 +73,4 @@ untyped-import
 untyped-type-import
 
 [version]
-^0.113.0
+^0.137.0