[ENH] Add left rhomboid

Mermaid have not only rhomboid `a[\"blah"\]` but the left version `b[/"blah"/]` (which is more commonly used to represent io operations) ```mermaid flowchart a[\"blah"\] b[/"blah"/] ``` From what I understand it should relatively easy to add this If maintainers don't have time for this, I can make a PR