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/typst/typst-show.typ

@@ -0,0 +1,17 @@
+// Civilytics — Typst show/entry partial for Quarto (typst-show.typ).
+// Pairs with typst-template.typ. Quarto appends the rendered document body
+// after this partial, so this file intentionally ends with the show rule and
+// no trailing body token. (Do not write that token in a comment here: Quarto
+// interpolates its template variables even inside comments.)
+// Title/subtitle/date are wrapped in [ ] so arbitrary text (including words
+// that are Typst keywords like "for"/"in") is treated as content, not code.
+// See civilyticsR issue #12.
+#show: doc => civilytics(
+  title: [$title$],
+  $if(subtitle)$subtitle: [$subtitle$],$endif$
+  $if(by-author)$authors: ($for(by-author)$"$it.name.literal$",$endfor$),$endif$
+  $if(date)$date: [$date$],$endif$
+  $if(abstract)$abstract: [$abstract$],$endif$
+  toc: $if(toc)$true$else$false$endif$,
+  doc
+)