handle xhtml void elements in mermaid diagrams

[bollwyvl]

References

• js port of Ensure void elements have closing slash in mermaid svg jupyterlab/jupyterlab#15661

Changes

• replace void HTML elements (like <br>) with their strict XHTML versions (<br />)

Examples

• Here is a gist of the two mermaid-related notebooks in the sibling repo

Future Work

• there are some upcoming changes needed to handle mermaid 10.8.0, which adds block diagrams
  • still being investigated on Upgrade to mermaid 11.x, marked 15.x jupyterlab/jupyterlab#15733

[blink1073]

Thank you!