Testing how to embed a filter inside a custom format.
The format-c definition in _extenison.yml needs to compile properly before we try to embed the filter. So first embed the filter with quarto add path/to/filter/ --embed format-c and only then insider filters: code into _extensions.yml for format-c.
The custom format yml should say:
contributes:
format:
html:
Note html not format-c or format-c-html.