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/cli/test.el

@@ -0,0 +1,48 @@
+;;; lisp/cli/test.el -*- lexical-binding: t; -*-
+;;; Commentary:
+;;
+;; The heart of Doom's test DSL and framework. Powered by either ERT or
+;; Buttercup, this extends testing frameworks to allow for isolated execution
+;; contexts on several levels, a more sophisticated CLI for tests, and
+;; integration with Doom's profiles system so testing environments can be
+;; generated on-the-fly.
+;;
+;;; Code:
+
+;;
+;;; Variables
+
+;; TODO Implement me
+(defvar doom-test-backend 'ert
+  "One of `ert' or `buttercup'.")
+
+;; TODO Implement me
+(defvar doom-test-isolation-level nil
+  "Determines the testing strategy for tests.
+
+Should be one of:
+
+  nil    -- Run all tests in the same session.
+  file   -- Run each test file in isolated sessions.
+  group  -- Run each group of tests in isolated sessions.
+  t      -- Run each individual test in isolated sessions (very slow).")
+
+
+;;
+;;; Commands
+
+;; FIXME Will be fixed in v3.1
+(defstub! test
+    ((backend ("--ert" "--buttercup"))
+     (jobs    ("-j" "--jobs" int))
+     &rest targets)
+  "Run Doom unit tests.")
+
+
+;;
+;;; Helpers
+
+;; Nothing here yet
+
+(provide 'doom-cli-test)
+;;; test.el ends here