Add support for exporting dynamic views to Mermaid sequence diagrams

[simonbrowndotje]

The PlantUML exporter can export dynamic views to a free format diagram and a sequence diagram, but the Mermaid exporter only supports a free format diagram. Mermaid does support sequence diagrams though, so perhaps this should be added. I don't know how many people are using the Mermaid exports, so add a reaction/comment if you'd like to see this.