feat(mu4e): add mu4e-compat package

(And remove vestigial ref to mu4e-alert in docs)

Fix: #6906
Fix: #7860

modules/email/mu4e/README.org

@@ -46,7 +46,7 @@ This module makes Emacs an email client, using [[https://www.djcbsoftware.nl/cod
   export) and HTML message.
 
 ** Packages
-- [[doom-package:mu4e-alert]]
+- [[doom-package:mu4e-compat]]
 - [[doom-package:org-msg]] if [[doom-module:+org]]
 
 ** TODO Hacks

modules/email/mu4e/config.el

@@ -24,6 +24,14 @@
         (lambda (&rest _)
           (expand-file-name ".attachments" (mu4e-root-maildir))))
   :config
+  ;; Ensures backward/forward compatibility for mu4e, which is prone to breaking
+  ;; updates, and also cannot be pinned, because it's bundled with mu (which you
+  ;; must install via your OS package manager).
+  (with-demoted-errors "%s" (require 'mu4e-compat nil t))
+  ;; For users on older mu4e.
+  (unless (boundp 'mu4e-headers-buffer-name)
+    (defvar mu4e-headers-buffer-name "*mu4e-headers*"))
+
   (cond ((or (modulep! +mbsync)
              (eq +mu4e-backend 'mbsync))
          (setq mu4e-get-mail-command "mbsync -a"

modules/email/mu4e/packages.el

@@ -1,5 +1,8 @@
 ;; -*- no-byte-compile: t; -*-
 ;;; email/mu4e/packages.el
 
+(package! mu4e-compat
+  :recipe (:host github :repo "tecosaur/mu4e-compat")
+   :pin "a33345cb8ef83554f01510bbc8f5c7323713aa8d")
 (when (modulep! +org)
   (package! org-msg :pin "59e2042e5f23e25f31c6aef0db1e70c6f54f117d"))