Add support for using KiCad V6 symbol libraries with generate_svg().

[devbisme]

    Well, I found the reason for this error: the drawing commands for KiCad V6 symbols are not processed so there's nothing for the SVG generator to work on. This is a larger fix than I can do right now, so your only immediate fix is to drop back and use KiCad V5 libraries. Sorry!

Originally posted by @devbisme in https://github.com/devbisme/skidl/issues/166#issuecomment-1314017610