gptel: Support gptel-mode in text-derived modes (#454)

* gptel.el (gptel-mode): gptel-mode can now be used in all modes
derived from `text-mode'.  This does not make sense for many
modes, like yaml-mode (which is derived from text), but it is up
to the user to handle these cases.  No changes to the
defaults.

gptel.el

@@ -1162,8 +1162,7 @@ file."
     map)
   (if gptel-mode
       (progn
-        (unless (or (derived-mode-p 'org-mode 'markdown-mode)
-                    (eq major-mode 'text-mode))
+        (unless (derived-mode-p 'org-mode 'markdown-mode 'text-mode)
           (gptel-mode -1)
           (user-error (format "`gptel-mode' is not supported in `%s'." major-mode)))
         (add-hook 'before-save-hook #'gptel--save-state nil t)