add a companion css editor section

[peterdd]

a second editor section for editing a css (file) similiar to other live coding editors like https://jsfiddle.net/

https://stackoverflow.com/questions/69219420/css-stylesheets-in-graphviz

Having SVG+CSS simplifies the styling of the results by using class attribute for nodes and edges.

[magjac]

Sorry for the delay and thanks for the suggestion. I understand that that might be useful if you use a stylesheet. I haven't used that myself though. I normally use the default attribute settings for nodes and edges to accomplish similar things, but perhaps that is not flexible enough?