Improving the mermaid-to-RDF parser

## I expect

- improve the mermaid to RDF parser in https://github.com/par-tec/d3fend-tools/blob/main/d3fendtools/mermaidrdf/__init__.py#L54
- support the full-fledged mermaid graph/flowchart syntax

## Instead

- some syntaxes are not supported

## Notes

- improve the filtering functionalities