Move VirtualizedList_EXPERIMENTAL to ES6 import/export

Summary:
VirtualizedList uses a combination of ES6 and CommonJS imports/exports. This moves fully to ES6 imports and exports, in the experimental version (which should soon replace the original).

Changelog:
[Internal][Changed] - Move VirtualizedList_EXPERIMENTAL to ES6 import/export

Reviewed By: rshest

Differential Revision: D39648806

fbshipit-source-id: a5da40f62a6010cd5b9894b02a91af86d3a8b995