Introduce new defcustom for terminal CUA rectangle commands

This allows a user to set a non-meta modifier for their terminal
should his/her terminal support it. See bug#35058 for background on
this change.

* lisp/emulation/cua-base.el (cua-rectangle-terminal-modifier-key):
New defcustom.

* lisp/emulation/cua-base.el (cua--shift-control-x-prefix): Use new
defcustom.

etc/NEWS

@@ -1252,6 +1252,13 @@ near the current column in Tabulated Lists (see variables
 +++
 *** 'text-mode-variant' is now obsolete, use 'derived-mode-p' instead.
 
+** CUA mode
+
+---
+*** New defcustom 'cua-rectangle-terminal-modifier-key'.
+This defcustom allows for the customization of the modifier key used
+in a terminal frame.
+
 
 * New Modes and Packages in Emacs 27.1
 

lisp/emulation/cua-base.el

@@ -427,7 +427,7 @@ and after the region marked by the rectangle to search."
 
 (defcustom cua-rectangle-modifier-key 'meta
   "Modifier key used for rectangle commands bindings.
-On non-window systems, always use the meta modifier.
+On non-window systems, use `cua-rectangle-terminal-modifier-key'.
 Must be set prior to enabling CUA."
   :type '(choice (const :tag "Meta key" meta)
 		 (const :tag "Alt key" alt)
@@ -435,6 +435,16 @@ Must be set prior to enabling CUA."
 		 (const :tag "Super key" super))
   :group 'cua)
 
+(defcustom cua-rectangle-terminal-modifier-key 'meta
+  "Modifier key used for rectangle commands bindings in terminals.
+Must be set prior to enabling CUA."
+  :type '(choice (const :tag "Meta key" meta)
+		 (const :tag "Alt key" alt)
+		 (const :tag "Hyper key" hyper)
+		 (const :tag "Super key" super))
+  :group 'cua
+  :version "27.1")
+
 (defcustom cua-enable-rectangle-auto-help t
   "If non-nil, automatically show help for region, rectangle and global mark."
   :type 'boolean
@@ -1237,10 +1247,9 @@ If ARG is the atom `-', scroll upward by nearly full screen."
 (defun cua--init-keymaps ()
   ;; Cache actual rectangle modifier key.
   (setq cua--rectangle-modifier-key
-	(if (and cua-rectangle-modifier-key
-		 (memq window-system '(x)))
-	    cua-rectangle-modifier-key
-	  'meta))
+	(if (eq (framep (selected-frame)) t)
+	    cua-rectangle-terminal-modifier-key
+	  cua-rectangle-modifier-key))
   ;; C-return always toggles rectangle mark
   (define-key cua-global-keymap cua-rectangle-mark-key	'cua-set-rectangle-mark)
   (unless (eq cua--rectangle-modifier-key 'meta)