Add JSX Fragment syntax support (#6552)

* Add JSX Fragments to babel-types

* Support JSX fragments in the transform-react-jsx plugin

* Add tests JSX fragments

* Update helper-builder and transform plugin documentations for jsx fragment

* Add generator for jsx fragments

* Add test for jsx fragment generator

* Split jsx transform example into normal and fragment examples

* Remove unnecessary fields from ElementState in babel-helper-builder-react-jsx

* inline [skip ci]

packages/babel-plugin-transform-react-jsx-compat/src/index.js

@@ -23,6 +23,7 @@ export default function({ types: t }) {
           );
         }
       },
+      compat: true,
     }),
   };
 }