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/shell/siren-shell-pop.el

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

modules/shell/siren-vterm.el

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

modules/shell/siren-xterm-color.el

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