Enhancement: add option to use Mermaid with DiagrammeR to display more complex Crow's Foot Notation ERDs

[SugarRayLua]

Since the dm package uses DiagrammeR to draw its diagrams and DiagrammeR can use the mermaid-js package which can draw more complex Crow's Foot Notation ERDs:

https://mermaid.js.org/syntax/entityRelationshipDiagram.html

and which DiagrammeR can currently incorporate through this update:

rich-iannone/DiagrammeR#475

it would be great if in the future the dm package could include an option to draw more complex Crow's Foot Notation ERDs using the DiagrammeR mermaid() function.

Thanks! :-)