Support mermaid syntax highlighting

[Yukaii]

https://github.com/bpruitt-goddard/vscode-mermaid-syntax-highlight/blob/master/syntaxes/mermaid.tmLanguage.yaml

https://github.com/mermaid-js/mermaid-live-editor/blob/master/src/mermaid-language/mermaid.js