Improve template styling via overridable Typst functions

[istathar]

Redesign the rendering pipeline so that Rust walks the domain model and emits calls to thin, individually overridable Typst functions. This replaces the previous code which produced a single large Typst literal and then required the user's template code to process that data structure into markup.

This involved making a change to where the tree-walk of that structure happened. While we were able to do so in Typst (#91) the result was that iterating on the default template was difficult and providing for user overrides of style elements necessitated the user to clone the default template containing machinery that really should have been internal.

Instead we further strengthen the idea of domain to include both the domain model created by the Adapter and the Renderer able to walk such a domain model on the Rust side. Rather than emitting a data dictionary literal, or finished Typst markup, we instead emit a stream of function calls: #render-description("Hello")#render-structure(":") and so on, with the function names matching the entities the domain's object model.

The functions are initially defined in a built-in template that is copied to .domain.typ in the directory containing the document to be rendered. If a custom user-defined template is provided via --template then an #include * directive for it is added to the top of the generated document, allowing that template to selectively override any of the individual render functions without having to reimplement the entire rendering pipeline.