Allow sorting stories in export order

[rwaltenberg]

Is your feature request related to a problem? Please describe.
I'm coming from Storybook, where a common pattern is to have a "Default" or "Playground" story for each component, followed by other stories with customizations. The "Default" story is always shown first when clicking on the component name because stories are sorted in the order they are exported in the *.stories.ts file.

Describe the solution you'd like
Currently, the storyOrder config allows specifying an array of stories or a function that returns an array. My suggestion is to support two additional string values:

• alphabetical – This is the current default behavior.
• export – This would allow sorting stories in the same order they are exported in the file.