refactor!: restructure Doom core

BREAKING CHANGE: This restructures the project in preparation for Doom
to be split into two repos. Users that have reconfigured Doom's CLI
stand a good chance of seeing breakage, especially if they've referred
to any core-* feature, e.g.

  (after! core-cli-ci ...)

To fix it, simply s/core-/doom-/, i.e.

  (after! doom-cli-ci ...)

What this commit specifically changes is:
- Renames all core features from core-* to doom-*
- Moves core/core-* -> lisp/doom-*
- Moves core/autoloads/* -> lisp/lib/*
- Moves core/templates -> templates/

Ref: #4273

lisp/lib/process.el

@@ -0,0 +1,42 @@
+;;; lisp/lib/process.el -*- lexical-binding: t; -*-
+
+;;;###autoload
+(defun doom-call-process (command &rest args)
+  "Execute COMMAND with ARGS synchronously.
+
+Returns (STATUS . OUTPUT) when it is done, where STATUS is the returned error
+code of the process and OUTPUT is its stdout output."
+  (with-temp-buffer
+    (cons (or (apply #'call-process command nil t nil (remq nil args))
+              -1)
+          (string-trim (buffer-string)))))
+
+;;;###autoload
+(defun doom-exec-process (command &rest args)
+  "Execute COMMAND with ARGS synchronously.
+
+Unlike `doom-call-process', this pipes output to `standard-output' on the fly to
+simulate 'exec' in the shell, so batch scripts could run external programs
+synchronously without sacrificing their output.
+
+Warning: freezes indefinitely on any stdin prompt."
+  ;; FIXME Is there any way to handle prompts?
+  (with-temp-buffer
+    (cons (let ((process
+                 (make-process :name "doom-sh"
+                               :buffer (current-buffer)
+                               :command (cons command (remq nil args))
+                               :connection-type 'pipe))
+                done-p)
+            (set-process-filter
+             process (lambda (_process output)
+                       (princ output (current-buffer))
+                       (princ (doom--format output))))
+            (set-process-sentinel
+             process (lambda (process _event)
+                       (when (memq (process-status process) '(exit stop))
+                         (setq done-p t))))
+            (while (not done-p)
+              (sit-for 0.1))
+            (process-exit-status process))
+          (string-trim (buffer-string)))))