feat(cli): introduce DOOMPROFILELOAD{FILE,PATH} envvars

- Adds $DOOMPROFILELOADFILE: Controls where to read and write the
  profile loader. Changing this may be helpful for users on nix/guix,
  who have deployed Doom to a read-only location. This sets
  `doom-profile-load-file`.
- Changed profile load file's default location (used to be
  $EMACSDIR/profiles/init.el, is now $EMACSDIR/profiles/load.el). The
  gitignore was updated to reflect this.
- Adds $DOOMPROFILELOADPATH: A colon-delimited list of profile config
  files and directories (semi-colon on Windows) which dictate what Doom
  reads in order to discover your profiles. Config files are required to
  have an *.el extension. This sets `doom-profile-load-path`.
- Changes the nomenclature around this loader script. I used to refer to
  it as the profile bootstrapper. I'll now refer to it as the profile
  load file, and I've renamed `doom-profiles-bootstrap-file` to
  `doom-profile-load-file` to reflect this.
- The variables `doom-profile-dirs` and `doom-profile-config-files` were
  merged into doom-profile-load-path.
- Both envvars have also been documented in `doom help` (and
  $DOOMPROFILE's has been updated).

Ref: #6794

early-init.el

@@ -74,14 +74,19 @@
              (setq user-emacs-directory (expand-file-name init-dir))))
        ;; FIX: Discard the switch to prevent "invalid option" errors later.
        (push (cons "--profile" (lambda (_) (pop argv))) command-switch-alist)
-       ;; Running 'doom sync' will (re)generate a lightweight profile
-       ;; bootstrapper in $EMACSDIR/profiles/init.el, after reading
-       ;; $EMACSDIR/profiles.el, $DOOMDIR/profiles,
-       ;; $XDG_CONFIG_HOME/doom-profiles.el, and ~/.doom-profiles.el. All it
-       ;; needs is for `$DOOMPROFILE' to be set.
+       ;; Running 'doom sync' or 'doom profile sync' (re)generates a light
+       ;; profile loader in $EMACSDIR/profiles/load.el (or
+       ;; $DOOMPROFILELOADFILE), after reading `doom-profile-load-path'. This
+       ;; loader requires `$DOOMPROFILE' be set to function.
        (setenv "DOOMPROFILE" profile)
-       (or (load (expand-file-name (format "profiles/init.%d.elc" emacs-major-version)
-                                   user-emacs-directory)
+       (or (load (expand-file-name
+                  (format (let ((lfile (getenv-internal "DOOMPROFILELOADFILE")))
+                            (if lfile
+                                (concat (string-remove-suffix ".el" lfile)
+                                        ".%d.elc")
+                              "profiles/load.%d.elc"))
+                          emacs-major-version)
+                  user-emacs-directory)
                  'noerror (not init-file-debug) 'nosuffix)
            (user-error "Profiles not initialized yet; run 'doom sync' first"))))