Support Mermaid diagrams

## Description
GitHub now supports Mermaids, like this:
```mermaid
graph TD;
A-->B;
A-->C;
B-->D;
C-->D;
```

You can more find about this in the official blog article: [Include diagrams in your Markdown files with Mermaid](https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/).

Therefore, more and more people will use Mermaid diagrams in their REAMDE.md files. It would be nice if pub.dev could also render Mermaid diagrams like GitHub it can.

Currently, it's not possible as you can see here: https://pub.dev/packages/nils_pub_dev_playground

| GitHub | pub.dev |
|-|-|
|  |  |