windows: Make ctrl-n open a new terminal when in a terminal (#38900)

This is how `ctrl-n` works on macOS. Right now `ctrl-n` on Windows with
the default keymap usually causes a new buffer to open, which is
inconvenient.

Release Notes:

- N/A

assets/keymaps/default-windows.json

@@ -1110,6 +1110,7 @@
       "alt-f": ["terminal::SendText", "\u001bf"],
       "alt-.": ["terminal::SendText", "\u001b."],
       "ctrl-delete": ["terminal::SendText", "\u001bd"],
+      "ctrl-n": "workspace::NewTerminal",
       // Overrides for conflicting keybindings
       "ctrl-b": ["terminal::SendKeystroke", "ctrl-b"],
       "ctrl-c": ["terminal::SendKeystroke", "ctrl-c"],