I have some custom wikitext(which also includes templates) and need to convert to text.
To do this, I use wikiexpand.

In the README file it is indicated:

Modules written in Lua and executed using {{#invoke:}} are not recognised, but can be replaced by implementing callable templates (that is, functions that render Wikicode)

The documentation in wikiexpand.expand.templates.TemplateStore.callable_templates is not very explicit.

Would it be possible to help me by giving me an example implementation?

For example, for the following wikicode:

{{ISBN|978-2-84066-599-1}}
{{Date de naissance|11|novembre|1866}}