Layout: Use type-only export to export ResponsiveSpacing.

ResponsiveSpacing is only a type, so exporting it in a JavaScript
context causes an import error when attempting to import glints-aries.
By using type-only exports, this line will be removed when transpiling
to JavaScript.

Author: yjwong

src/index.ts

@@ -15,7 +15,7 @@ export { default as Drawer } from './Navigation/Drawer';
 export { default as Dropdown } from './Navigation/Dropdown';
 export { default as Gallery } from './Display/Gallery';
 export { Box } from './Layout/Box';
-export { ResponsiveSpacing } from './Layout/Box/types';
+export type { ResponsiveSpacing } from './Layout/Box/types';
 export { Flex } from './Layout/Flex';
 export { default as Grid } from './Layout/Grid';
 export { default as Heading } from './General/Heading';