feat(version-control): correctly format forge-post-mode buffers with prettier-js

forge-post-mode uses temporary files for editing, which means that
prettier can't determine the formatter to use based on the
filename. Hence we need to explicitly pass `--parser markdown` as a CLI
arg to prettier.

modules/version-control/siren-forge.el

@@ -12,5 +12,18 @@
   :defer t
   :after magit)
 
+(use-package forge-post
+  :straight forge
+  :defer t
+
+  :hook
+  (forge-post-mode . siren-forge-post-mode-setup)
+
+  :init
+  (defun siren-forge-post-mode-setup ()
+    (setq-local prettier-js-args '("--parser" "markdown"
+                                   "--print-width" "80"
+                                   "--prose-wrap" "always"))))
+
 (provide 'siren-forge)
 ;;; siren-forge.el ends here