Expose enableBabelRuntime config param externally

Summary:
Up until now the `enableBabelRuntime` option in the `metro-react-native-preset` package was hardcoded in the transformer: our internal transformer set it to `false` and the opensource one was `true`.

This diff now exposes this param out to the config of Metro (now that it's quite easy to expose things) so anybody can set it to false or true.

This allows our end to end tests to set it to false to not have to deal with requires to `babel/runtime`.

Reviewed By: davidaurelio

Differential Revision: D10123776

fbshipit-source-id: 97e744eeb567c8e86ab3c613ac71970d226ed2ca