Support elbow arrows via flowchart line curve styling

[mtolmacs]

Line curve style pragma definition for flowcharts

Elbow arrows could be represented by the "stepAfter" style.

Elbow arrows could be created via the Excalidraw API when creating a new "arrow" element (element.type) and setting the attribute "elbowed" to "true". The "elbowed" attribute only exists on "arrow" type elements, not on linear elements.