Pin to Mermaid v9

[seshrs]

Context

Closes eecs280staff/p5-ml#183.

@amirkamil noticed that Mermaid diagrams weren't rendering on the P5 spec:

Expected	Actual

Digging a bit, I found mermaid-js/mermaid#4442, which noted that the root cause was a breaking change in v10 of MermaidJS:

Mermaid is ESM only!

I didn't notice when adding MermaidJS support to Primer Spec, but it turned out that the documentation had let me to always use the latest version of MermaidJS instead of pinning to a specific version — hence, we were exposed to the breaking change :(

In this PR

This commit changes the Primer Spec to always use MermaidJS v9, which is compatible with CJS and exposes the global variable mermaid.

In future, it would make sense to follow-up how best to upgrade to Mermaid v10. But this works for now.

Validation

Visit the preview URL's "Diagrams" demo. Notice that the Mermaid diagrams now render again.

[github-actions]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/254/.

^{(Available until Fri Dec 08 2023.)}

[awdeorio]

In future, it would make sense to follow-up how best to upgrade to Mermaid v10. But this works for now.

If you feel this is something that's important to track, consider making an issue :)