Integrate Mermaid diagrams

As I was suggesting on Mastodon, it would be cool to integrate JavaScript-based diagramming functionality in addition to currently available MathJax into WriteFreely :nerd_face:

While using Gitea, a super-nice Git server, I noticed text-based (therefore easy to version) diagramming solution called MermaidJS - see it in action here.

Since we currently have MathJax which we can use in WriteFreely posts like this:

$\( ax^2 + \sqrt{bx} + c = 0 \)$

While we post code to be highlighted like this (mentioning language, like “sql” here, too):

```sql
SELECT version ();
```

The way how Gitea does if you specify “mermaid” as language in code block, it will render a diagram. Therefore posting:

```mermaid
sequenceDiagram
    Alice->>John: Hello John, how are you?
    John-->>Alice: Great!
    Alice-)John: See you later!
```

Would render a diagram like this.

Do you see such functionality as useful?

MermaidJS could be enabled as per-blog setting under “Text Rendering” settings :nerd_face: