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.

core/core-ui.el

@@ -522,7 +522,8 @@ windows, switch to `doom-fallback-buffer'. Otherwise, delegate to original
 ;; User themes should live in $DOOMDIR/themes, not ~/.emacs.d
 (setq custom-theme-directory (concat doom-private-dir "themes/"))
 
-;; Always prioritize the user's themes above the built-in/packaged ones.
+;; Third party themes add themselves to `custom-theme-load-path', but the themes
+;; living in $DOOMDIR/themes should always have priority.
 (setq custom-theme-load-path
       (cons 'custom-theme-directory
             (delq 'custom-theme-directory custom-theme-load-path)))
@@ -593,6 +594,7 @@ windows, switch to `doom-fallback-buffer'. Otherwise, delegate to original
         (when (and (not no-enable) (custom-theme-enabled-p theme))
           (setq doom-theme theme)
           (put 'doom-theme 'previous-themes (or last-themes 'none))
+          ;; DEPRECATED Hook into `enable-theme-functions' when we target 29
           (doom-run-hooks 'doom-load-theme-hook))))))