fix(typst): ship template as partials so code blocks render (#12)

The single-file `template: civilytics-typst.typ` discarded Quarto's
auto-generated `definitions` partial, so any Typst document containing a
code block failed with `unknown variable: Skylighting`.

Ship the template as Quarto template-partials instead, so Quarto keeps its
definitions (Skylighting + token functions) and syntax-highlighted code
blocks render while the Civilytics branding still applies:

- split civilytics-typst.typ into typst-template.typ (the styling function)
  and typst-show.typ (the show/entry point, keyword-safe [ ] wrapping kept);
  remove the single-file template
- use_civilytics_theme() now copies both partials and prints the
  template-partials usage
- example report.qmd uses template-partials

Verified: a report with an R code block renders with working syntax
highlighting and full branding.

inst/quarto/examples/report.qmd

@@ -19,7 +19,9 @@ format:
     toc: true
     toc-location: right
   typst:
-    template: ../typst/civilytics-typst.typ
+    template-partials:
+      - ../typst/typst-template.typ
+      - ../typst/typst-show.typ
   pdf:
     include-in-header: ../latex/civilytics.tex
     include-before-body: ../latex/civilytics-title.tex