Migrate SVG export code

We currently have code to export diagrams to an SVG string [in the main extension repository](https://github.com/EDAcation/vscode-edacation/blob/df8ca52f5d8d283996e2312009c872c5a793bf98/views/digitaljs/src/main.ts#L25-L42). This should probably be migrated and integrated into this fork. Make sure that sub-circuits can also be exported.
