Change default pane keybindings to duplicate, rather than use default…

… profile (#16951)

As we discussed in length over at #7657. 

This changes the default <kbd>alt+shift+-</kbd> and
<kbd>alt+shift+plus</kbd> keybindings to split panes by duplicating the
pane by default.

Closes #7657

src/cascadia/TerminalSettingsModel/defaults.json

@@ -484,9 +484,11 @@
         { "command": "closeOtherPanes" },
         { "command": "closePane", "keys": "ctrl+shift+w" },
         { "command": { "action": "splitPane", "split": "up" } },
-        { "command": { "action": "splitPane", "split": "down" }, "keys": "alt+shift+-" },
+        { "command": { "action": "splitPane", "split": "down" } },
         { "command": { "action": "splitPane", "split": "left" } },
-        { "command": { "action": "splitPane", "split": "right" }, "keys": "alt+shift+plus" },
+        { "command": { "action": "splitPane", "split": "right" }  },
+        { "command": { "action": "splitPane", "splitMode": "duplicate", "split": "down" }, "keys": "alt+shift+-" },
+        { "command": { "action": "splitPane", "splitMode": "duplicate", "split": "right" }, "keys": "alt+shift+plus" },
         { "command": { "action": "resizePane", "direction": "down" }, "keys": "alt+shift+down" },
         { "command": { "action": "resizePane", "direction": "left" }, "keys": "alt+shift+left" },
         { "command": { "action": "resizePane", "direction": "right" }, "keys": "alt+shift+right" },