feat: backport --init-directory for 27/28 users

--init-directory was added in Emacs 29. This backports it for 27/28
users, so users can trivially load an Emacs config living in another
directory (will be necessary for future versions of Doom's sandbox).
However, for this to work, Doom needs to live in ~/.emacs.d or
~/.config/emacs.

Ref: emacs-mirror/emacs@8eaf04de83

early-init.el

@@ -69,11 +69,28 @@
 ;;
 ;;; Bootstrap
 
-;; Ensure Doom is running out of this file's directory
+(let (;; DEPRECATED Backported from 29. Remove when 27/28 support is removed.
-(setq user-emacs-directory (file-name-directory load-file-name))
+      (initdir (cadr (member "--init-directory" command-line-args)))
+
+      initfile)
+
+  ;; But discard the switches later to prevent "invalid option" errors.
+  (when initdir
+    (add-to-list 'command-switch-alist (cons "--init-directory" (lambda (_) (pop argv)))))
+
+  ;; Detect emacs directory
+  (setq user-emacs-directory
+        (cond (initdir (expand-file-name initdir))
+              ((getenv-internal "EMACSDIR"))
+              (user-emacs-directory)))
 
   ;; Load the heart of Doom Emacs
-(load (concat user-emacs-directory "core/core") nil 'nomessage)
+  (if (load (expand-file-name "core/core" user-emacs-directory) t t)
+      ;; ...and prepare it for an interactive session.
+      (setq initfile (expand-file-name "core-start" doom-core-dir))
+    ;; ...but if that fails, then assume this isn't a Doom config.
+    (setq early-init-file (expand-file-name "early-init" user-emacs-directory))
+    (load early-init-file t t))
 
   ;; We hijack Emacs' initfile resolver to inject our own entry point. Why do
   ;; this? Because:
@@ -94,8 +111,9 @@
   (define-advice startup--load-user-init-file (:filter-args (args) init-doom)
     "Initialize Doom Emacs in an interactive session."
     (list (lambda ()
-          (expand-file-name "core-start" doom-core-dir))
+            (or initfile
+                (expand-file-name "init.el" user-emacs-directory)))
           nil  ; TODO Replace with safe mode initfile
-        (caddr args)))
+          (caddr args))))
 
 ;;; early-init.el ends here