refactor: how Doom starts up

Restructures Doom's primary core files and entry points to prepare for
backports (from the new CLI) coming soon.

- Removes $EMACSDIR/init.el.
- Doom configures Emacs to ignore ~/.emacs and ~/_emacs files.
- Doom's bootstrapper for interactive sessions was moved out of core.el
  and doom-initialize into doom-start.el. This change is preparation for
  Doom's new profile system (coming soon™️), where this bootstrapper
  will be dynamically generated.
- core.el and early-init.el have been reorganized, comment headers moved
  around, and comments updated to reflect these changes.

early-init.el

@@ -1,23 +1,23 @@
 ;;; early-init.el -*- lexical-binding: t; -*-
-
+;;; Commentary:
+;;
 ;; Emacs 27.1 introduced early-init.el, which is run before init.el, before
-;; package and UI initialization happens, and before site files are loaded.
+;; package and UI initialization happens, and before site files are loaded. This
+;; is the best time to make all our changes (though any UI work will have to be
+;; deferred).
+;;
+;; This file is the entry point into Doom Emacs for your standard, interactive
+;; Emacs session, and houses our most pressing and hackiest startup
+;; optimizations. That said, only the "bootstrap" section at the bottom is
+;; required for Doom to function.
+;;
+;;; Code:
 
 ;; A big contributor to startup times is garbage collection. We up the gc
 ;; threshold to temporarily prevent it from running, then reset it later by
 ;; enabling `gcmh-mode'. Not resetting it will cause stuttering/freezes.
 (setq gc-cons-threshold most-positive-fixnum)
 
-;; Prevent unwanted runtime compilation for gccemacs (native-comp) users;
-;; packages are compiled ahead-of-time when they are installed and site files
-;; are compiled when gccemacs is installed.
-(setq native-comp-deferred-compilation nil)
-
-;; In Emacs 27+, package initialization occurs before `user-init-file' is
-;; loaded, but after `early-init-file'. Doom handles package initialization, so
-;; we must prevent Emacs from doing it early!
-(setq package-enable-at-startup nil)
-
 ;; In noninteractive sessions, prioritize non-byte-compiled source files to
 ;; prevent the use of stale byte-code. Otherwise, it saves us a little IO time
 ;; to skip the mtime checks on every *.elc file.
@@ -69,15 +69,33 @@
 ;;
 ;;; Bootstrap
 
-;; Contrary to what many Emacs users have in their configs, you don't need
-;; more than this to make UTF-8 the default coding system:
-(set-language-environment "UTF-8")
-
-;; set-language-enviornment sets default-input-method, which is unwanted
-(setq default-input-method nil)
-
 ;; Ensure Doom is running out of this file's directory
 (setq user-emacs-directory (file-name-directory load-file-name))
 
 ;; Load the heart of Doom Emacs
 (load (concat user-emacs-directory "core/core") nil 'nomessage)
+
+;; We hijack Emacs' initfile resolver to inject our own entry point. Why do
+;; this? Because:
+;;
+;; - It spares Emacs the effort of looking for/loading useless initfiles, like
+;;   ~/.emacs and ~/_emacs. And skips ~/.emacs.d/init.el, which won't exist if
+;;   you're using Doom (fyi: doom hackers or chemacs users could then use
+;;   $EMACSDIR as their $DOOMDIR, if they wanted).
+;; - Later, 'doom sync' will dynamically generate its bootstrap file, which is
+;;   important for Doom's soon-to-be profile system (which can replace Chemacs).
+;;   Until then, we'll use core/core-start.el.
+;; - A "fallback" initfile can be trivially specified, in case the bootstrapper
+;;   is missing (if the user hasn't run 'doom sync' or is a first-timer). This
+;;   is an opportunity to display a "safe mode" environment that's less
+;;   intimidating and more helpful than the broken state errors would've left
+;;   Emacs in, otherwise.
+;; - A generated config allows for a file IO optimized startup.
+(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))
+        nil  ; TODO Replace with safe mode initfile
+        (caddr args)))
+
+;;; early-init.el ends here