Add language support for Mermaid

[cahilfoley]

Language
Mermaid is a language for creating diagrams using text and code.

An example is:

graph TD
    A[Start] --> B{Is it?};
    B -->|Yes| C[OK];
    C --> D[Rethink];
    D --> B;
    B ---->|No| E[End];

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Which can be rendered into the diagram:

Additional resources

• Official website
• Documentation
• Git Repo