plot.GRiwrm: use web service instead of DiagrammeR::mermaid

3 caveats are driving this change request:

• DiagrammeR::mermaid produces a HTML widget which is very hard to render in PDF (https://bookdown.org/yihui/rmarkdown-cookbook/html-widgets.html)
• DiagrammeR implementation of mermaid is old and bugged and there are no clean solution yet for updating it (https://github.com/rich-iannone/DiagrammeR/issues/475)
• DiagrammeR has to much dependencies for just using mermaid

I propose to use the solution adopted in fairify (https://forgemia.inra.fr/umr-g-eau/fairify/-/blob/main/R/mermaid.R?ref_type=heads) which use the service provided by https://mermaid.live with a fall back to DiagrammeR::mermaid if the web service is unavailable.