Move storySort from preview.js to main.js

[shilman]

Describe the bug

Currently sorting stories in the sidebar is done using a parameter, options.storySort because once upon a time that code was executed in the preview.

Today, sorting is done in Node. That means that:

1. storySort can only be defined in preview.js, making it a weird parameter
2. It must be statically extracted using AST techniques
3. It has various constraints due to Add support for custom webpack config #2

This is pure tech debt. We should:

• Move storySort to main.js
• Update the index and any referencing code
• Create an automigration to move existing storySort configs to main.js
• Update the documentation

This is a breaking change that can only be done in a major release.