fix(elisp): add lexical-binding comment to all files to suppress new Emacs 31 warnings

I've set `lexical-binding` to `nil` in all Emacs Lisp files to suppress
the warnings introduced in Emacs 31 requiring all elisp files to have a
`lexical-binding` comment.

This retains the default behavior of dynamic binding when no
`lexical-binding` comment is present. With it set to `t` across the
board, various things break, and fixing those is a task for another day.

modules/documentation/siren-dash-at-point.el

@@ -1,4 +1,4 @@
-;;; siren-dash-at-point.el --- jimeh's Emacs Siren: dash-at-point configuration.
+;;; siren-dash-at-point.el --- jimeh's Emacs Siren: dash-at-point configuration.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

modules/documentation/siren-helpful.el

@@ -1,4 +1,4 @@
-;;; siren-helpful.el --- jimeh's Emacs Siren: helpful configuration.
+;;; siren-helpful.el --- jimeh's Emacs Siren: helpful configuration.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary: