Fix syntax highlighting

#2071 broke syntax highlighting because guides-sources fails to satisfy ember-shiki's peerDependency on showdown.

By not providing it, we left pnpm free to pick whatever copy it wanted, and thus there was no way to guarantee that ember-shiki and the rest of guides-source were using *the same* copy.

Author: ef4

package.json

@@ -110,6 +110,7 @@
     "retext-spell": "^4.0.0",
     "retext-syntax-urls": "^2.0.0",
     "shelljs": "^0.8.5",
+    "showdown": "^2.1.0",
     "unified": "^9.2.1",
     "walk-sync": "^2.0.2",
     "webpack": "^5.72.1"