Csf Tools: Fix overriding scalar named export values

[shilman]

Closes #21127

What I did

Update ConfigFile's setFieldNode method to handle setting named exports that are scalars

How to test

• See attached unit tests

sb automigrate bare-mdx-stories-glob

in a project with:

export const stories = ['./Introduction.stories.mdx', '../src/App.stories.tsx']

[yannbf]

Looks good for a hotfix, we probably need to rethink csf-tools logic if we want to account for many other scenarios. Here's another one, which I don't know if it's really common:

const stories = ['./Introduction.stories.mdx', '../src/App.stories.tsx'];
export { stories };

[shilman]

Note: this PR doesn't handle the following interesting cases raised by @yannbf

This one requires a significant refactor (see my FIXME comment) in the PR:

const stories = ['./Introduction.stories.mdx', '../src/App.stories.tsx'];
export { stories };

This one requires a change in the automigration if we want to support this feature (I vote not):

const autoStories = ['../src/components/atoms/**.stories.*'];
export const stories = ['./Introduction.stories.mdx', '../src/App.stories.tsx', ...autoStories]

Can't wait for the day when everybody is updated to default exports! 🙏

Cc @JReinhold