Add support for export * statements in remark-mdx

[zephraph]

Scope

Adds the ability to use an export * statement inside an mdx file.

Context

While working hashicorp/next-mdx-enhanced#37 I kept getting these remark eat errors.

Incorrectly eaten value: please report this warning on https://git.io/vg5Ft

Thanks to the playground I traced it down to the export statement I added to the above PR:

export * from 'blah'

Check out the above PR link for details about why I was trying to add this.

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[johno]

Awesome, thanks!