Fix automatic source code previews

[calebeby]

Fixes #1695

You can test patterns that have auto-generated source code previews, for example:

• https://deploy-preview-1733--cloudfour-patterns.netlify.app/?path=/docs/components-alert--basic
• https://deploy-preview-1733--cloudfour-patterns.netlify.app/?path=/docs/components-author--basic-usage

Tyler did most of the figuring out/fixing, and I just added a fallback for storyContext.initialArgs.

I did not fix the syntax highlighting, I don't remember how that works and I don't know how to fix it

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Latest commit: f885e24

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[tylersticka]

Thank you, @calebeby! Such a simple fix but I could not figure it out on my own.