Skip hermes-parser under Babel for non-Flow JS code

Summary:
Resolves facebook/hermes#1549.

Changelog:
[General][Fixed] - When using Babel with plain JavaScript files, support for additional user syntax plugins should be fixed (now uses Babel's parser instead of hermes-parser). There is no change for JS files annotated with `flow`, where extended JS syntax remains unsupported.

Differential Revision: D65816797

packages/react-native-babel-preset/src/configs/main.js

@@ -25,7 +25,7 @@ function isTSXSource(fileName) {
 const loose = true;
 
 const defaultPlugins = [
-  [require('babel-plugin-syntax-hermes-parser')],
+  [require('babel-plugin-syntax-hermes-parser'), {parseLangTypes: 'flow'}],
   [require('babel-plugin-transform-flow-enums')],
   [require('@babel/plugin-transform-block-scoping')],
   [require('@babel/plugin-transform-class-properties'), {loose}],