fix: enum export

[bvandercar-vt]

Since these were recently changed to be enums and types, we need to do export instead of export type so that we can use it as a constant.

Currently, when we try to do.

import { Size } from '@blueprintjs/core'

...

<Button>
   size={Size.LARGE}

We get the error "'Size' cannot be used as a value because it was exported using 'export type'."

[ggdouglas]

LGTM. Thanks for the catch!