Add syntax highlighting support for ruby and erb

[colebemis]

This PR adds support for ruby and erb syntax highlighting in code examples:

Example

```erb
<h1 class="hi">foo</h1>
<%= render(Primer::SpinnerComponent.new) %>
```

Before

After

Closes #200

[changeset-bot]

🦋 Changeset detected

Latest commit: 5709e7a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@primer/gatsby-theme-doctocat	Minor

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doctocat – ./

🔍 Inspect: https://vercel.com/primer/doctocat/7amR94d5iunr2ETZqBits1n8ND37
✅ Preview: https://doctocat-git-erb-syntax-primer.vercel.app

gatsby-theme-primer-example – ./

🔍 Inspect: https://vercel.com/primer/gatsby-theme-primer-example/4QGkwF2EbYdxkexZynaQ8EQkUd9a
✅ Preview: https://gatsby-theme-primer-example-git-erb-syntax-primer.vercel.app

[colebemis]

Looks like this is working in the Primer ViewComponents docs for one-line examples:

...but it has a hard time with multi-line examples:

[colebemis]

I filed an issue in the prism repo to report the bug described in my previous comment: PrismJS/prism#2602

[manuelpuyol]

@colebemis any updates here?

[colebemis]

@manuelpuyol Sorry, I haven't had time to follow up on this. If you feel comfortable, feel free to take over this PR. Otherwise, I'll revisit this when I have more time.

[manuelpuyol]

Dug some more and I think I found the problem FormidableLabs/prism-react-renderer#104

[manuelpuyol]

As an update, my PR FormidableLabs/prism-react-renderer#105 was merged! I'm waiting for prism-react-renderer to publish a new version with the fix.

For it to work properly we have to set up our custom PrismJS, which I did in theme/src/prism.js, but that means that we need to manually setup the languages we want to support. @colebemis mind helping me here how to proceed?

[manuelpuyol]

prism-react-rendered changed the github theme on v1.2.0 and it does not look right:

Before	After

So I decided to extract the previous version of the theme into this project to keep the docs consistent (keeping the Before style)

[colebemis]

Wow, nice work, @manuelpuyol! Very impressed with your debugging abilities. Thanks for pushing this over the finish line ❤️

Approved ✅ (I can't officially approve because I started the PR 😅)