feat(lang): ensure markdown formatting respects local fill-column width

Previously prettier would always format markdown files to 80 character
long lines. Now it reads the character width from the fill-column
variable in such a way that fill-column can be set with dir-local
variables.

modules/languages/siren-markdown.el

@@ -40,10 +40,12 @@
 
   :init
   (defun siren-markdown-mode-setup ()
+    ;; Configure prettier after local vars are processed, allowing local
+    ;; override of fill-column and have prettier respect it.
+    (add-hook 'hack-local-variables-hook
+              'siren-markdown-mode-setup-prettier nil t)
+
     (setq-local markdown-asymmetric-header t
-                prettier-js-args '("--parser" "markdown"
-                                   "--print-width" "80"
-                                   "--prose-wrap" "always")
                 whitespace-action nil)
 
     (siren-display-fill-column)
@@ -52,7 +54,12 @@
     (prettier-js-mode)
     (flyspell-mode)
     (smartparens-mode +1)
-    (subword-mode)))
+    (subword-mode))
+
+  (defun siren-markdown-mode-setup-prettier ()
+    (setq-local prettier-js-args `("--parser" "markdown"
+                                   "--print-width" ,(number-to-string fill-column)
+                                   "--prose-wrap" "always"))))
 
 ;; Required by markdown-edit-code-block.
 (use-package edit-indirect

modules/major-modes/siren-prog-mode.el

@@ -14,10 +14,13 @@
   (prog-mode . siren-prog-mode-setup)
 
   :init
+  (add-to-list 'safe-local-variable-values
+               '(fill-column . 120))
+
   (defun siren-prog-mode-setup ()
     "Default coding hook, useful with any programming language."
-    (setq fill-column 80
-          whitespace-action '(auto-cleanup))
+    (setq-local fill-column 80
+                whitespace-action '(auto-cleanup))
 
     (siren-display-indentation 1)
     (hl-line-mode t)

modules/major-modes/siren-text-mode.el

@@ -12,7 +12,7 @@
   :hook (text-mode . siren-text-mode-setup)
   :init
   (defun siren-text-mode-setup ()
-    (setq fill-column 80)
+    (setq-local fill-column 80)
 
     (hl-line-mode t)
     (visual-line-mode t)))