feat(lib): add mermaid diagram debug output + create issue template

[jeertmans]

Currently, the debug features outputs a diagram that it not very easy to visualize.

As demonstrated in #393, GitHub supports mermaid diagrams and I think this would be an excellent feature to add a debug feature that outputs the code for the corresponding mermaid diagram.

Also, this would make sense to create an issue template that asks for users to include to mermaid in the issues, so we can easily visualize what could be wrong!

• Implement some debug-mermaid features;
• Create an issue template that asks to input the mermaid diagram (when relevant).