Add mermaid directive support

[jakecrews74]

Currently the plugin does not handle mermaid directives. This is because the isMermaidCode check just checks the keyword the mermaid diagram starts with, which will always return false since Mermaid directives have to be first. I added handling to run the check against with the directive removed if included.

[johanneswuerbach]

Released as v0.6.0, thank you 🤩