fix(gatsby-starter-blog): Load PrismJS theme CSS

[YoshiWalsh]

Description

Currently this starter includes gatsby-remark-prismjs but doesn't load a theme for it. Loading a theme is considered a requirement by gatsby-remark-prismjs, since without one syntax highlighting doesn't work properly. This causes confusion for new users.

This PR loads the 'Coy' theme, which I think fits in nicely with the aesthetic of the starter.

I'm not sure if this starter is meant to be as minimal as possible or if it's meant to provide a best-practices starting point. If the latter, I think we should also include the line highlighting styles, line numbering styles, and fancy shell prompt. These don't require any additional NPM dependencies, but they do slightly increase the footprint of each page. I'm happy to also add these to the starter if people think it's a good idea.

Documentation

I don't believe there's any documentation required here. If we decide not to add the optional styles then maybe we should include a note about them in the instructions.

[LekoArts]

Thanks for the PR! I changed the commonJS import and used the default Prism theme.