Swap shiki for highlight.js's core build for syntax highlighting

Shiki pulls in a WASM oniguruma engine and bundled theme/grammar JSON
for two full themes; highlight.js's core build plus only the 11
languages content actually uses is much thinner for the same job here.
Token colors now come straight from the design tokens (--accent,
--text-muted, ...) instead of shiki's dual-theme CSS vars, so they
already switch with the site's theme with no extra [data-theme] rules.

apps/site/package.json

@@ -21,9 +21,9 @@
     "@fontsource-variable/newsreader": "^5.3.0",
     "@fontsource/jetbrains-mono": "^5.3.0",
     "astro": "^7.2.9",
+    "highlight.js": "^11.12.0",
     "marked": "^18.0.11",
     "pagefind": "^1.5.2",
-    "shiki": "^4.4.3",
     "smol-toml": "^1.8.0",
     "three": "^0.185.1"
   },