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/completion/siren-auto-complete.el

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

modules/completion/siren-cape.el

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

modules/completion/siren-company-prescient.el

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

modules/completion/siren-company.el

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

modules/completion/siren-copilot.el

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

modules/completion/siren-marginalia.el

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