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.

core/siren-core-compile.el

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

core/siren-core-custom.el

@@ -1,4 +1,4 @@
-;;; siren-core-custom.el --- jimeh's Emacs Siren: Siren's customizable variables.
+;;; siren-core-custom.el --- jimeh's Emacs Siren: Siren's customizable variables.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-editor.el

@@ -1,4 +1,4 @@
-;;; siren-core-editor.el --- jimeh's Emacs Siren: Editor settings.
+;;; siren-core-editor.el --- jimeh's Emacs Siren: Editor settings.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-env.el

@@ -1,4 +1,4 @@
-;;; siren-core-env.el --- jimeh's Emacs Siren: Environment variable setup.
+;;; siren-core-env.el --- jimeh's Emacs Siren: Environment variable setup.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-init.el

@@ -1,4 +1,4 @@
-;;; siren-core-init.el --- jimeh's Emacs Siren: Initialize all the things
+;;; siren-core-init.el --- jimeh's Emacs Siren: Initialize all the things  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-keybinds.el

@@ -1,4 +1,4 @@
-;;; siren-core-keybinds.el --- jimeh's Emacs Siren: Core keybinds support
+;;; siren-core-keybinds.el --- jimeh's Emacs Siren: Core keybinds support  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-linux.el

@@ -1,4 +1,4 @@
-;;; siren-core-linux.el --- jimeh's Emacs Siren: Linux specific settings.
+;;; siren-core-linux.el --- jimeh's Emacs Siren: Linux specific settings.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-macos.el

@@ -1,4 +1,4 @@
-;;; siren-core-macos.el --- jimeh's Emacs Siren: macOS specific settings.
+;;; siren-core-macos.el --- jimeh's Emacs Siren: macOS specific settings.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-modules.el

@@ -1,4 +1,4 @@
-;;; siren-core-modules.el --- jimeh's Emacs Siren: Module loading.
+;;; siren-core-modules.el --- jimeh's Emacs Siren: Module loading.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-package-overrides.el

@@ -1,4 +1,4 @@
-;;; siren-core-package-overrides.el --- jimeh's Emacs Siren: Package overrides
+;;; siren-core-package-overrides.el --- jimeh's Emacs Siren: Package overrides  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-packages.el

@@ -1,4 +1,4 @@
-;;; siren-core-packages.el --- jimeh's Emacs Siren: Core package setup
+;;; siren-core-packages.el --- jimeh's Emacs Siren: Core package setup  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-performance.el

@@ -1,4 +1,4 @@
-;;; siren-core-performance.el --- jimeh's Emacs Siren: Performance tweaks
+;;; siren-core-performance.el --- jimeh's Emacs Siren: Performance tweaks  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-themes.el

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

core/siren-core-ui.el

@@ -1,4 +1,4 @@
-;;; siren-core-ui.el --- jimeh's Emacs Siren: Emacs UI settings.
+;;; siren-core-ui.el --- jimeh's Emacs Siren: Emacs UI settings.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-utils.el

@@ -1,4 +1,4 @@
-;;; siren-core-utils.el --- jimeh's Emacs Siren: Core Siren functions and macros.
+;;; siren-core-utils.el --- jimeh's Emacs Siren: Core Siren functions and macros.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary:
 

core/siren-core-vendor.el

@@ -1,4 +1,4 @@
-;;; siren-core-vendor.el --- jimeh's Emacs Siren: Module loading.
+;;; siren-core-vendor.el --- jimeh's Emacs Siren: Module loading.  -*- lexical-binding: nil; -*-
 
 ;;; Commentary: