feat: add flowChart style to mermaid visualization

[julianrschmidt]

We needed a new way of displaying the state machine in mermaid style in order to highlight the current state. To my knowledge this cannot be easily done with the previously implemented StateDiagram type, which is why we implemented the far more flexible style FlowChart (LR).
The code is backward-compatible .
I would suggest to move the different visualizers (graphviz/mermaid (maybe more in the future)) into separate files, but did not want to do this before general acceptance of this feature.

[maxekman]

I would be fine with a restructure as it’s not part of the core package. Feel free to do it if you want and can spare the time!

[julianrschmidt]

@maxekman For now I would leave it like this and propose to merge it into the main project.
In a different merge request, I would like to restructure the visualizer files into a different package. This would require the fsm to publicly export a GetTransitions method, which also might be useful for others to write their own visualizers without having to do this in the fsm package. What do you think?

[maxekman]

Sounds good!

[maxekman]

LGTM, thanks for the contribution. 👍🏼

[julianrschmidt]

Thank you! Feel free to merge it. We are waiting to use it!