Copy linked files if they are not supported by transformer-sharp

[vladistan]

Thanks for providing great starter theme. Unfortunately, there is a small problem with the minimal-blog-core theme. It doesn't support animated gifs.

The post below explains the root cause of the issue

https://www.reddit.com/r/gatsbyjs/comments/hrs7tj/gatsby_not_rendering_gifs_in_markdown_files/

This Gatsby issue explains how to fix it

gatsbyjs/gatsby#7317

And this PR provides the fix. Because Gatsby theme shadowing does not extend to the config files the core theme has to be patched to provide the fix.

[netlify]

👷 Deploy request for gatsby-themes pending review.

Visit the deploys page to approve it

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