chore(lang): Add support for mermaid diagrams

The org-babal features of mermaid-mode itself don't seem to work, but
the separate ob-mermaid does work.

modules/languages/siren-mermaid.el

@@ -0,0 +1,22 @@
+;;; siren-mermaid.el --- jimeh's Emacs Siren: mermaid-mode configuration.
+
+;;; Commentary:
+
+;; Basic configuration for mermaid-mode.
+
+;;; Code:
+
+(require 'siren-prog-mode)
+
+(use-package mermaid-mode
+  :mode "\\.mermaid\\'" "\\.mmd\\'"
+
+  :hook
+  (mermaid-mode . siren-mermaid-mode-setup)
+
+  :init
+  (defun siren-mermaid-mode-setup ()
+    (run-hooks 'prog-mode-hook)))
+
+(provide 'siren-mermaid)
+;;; siren-mermaid.el ends here