terminal: Bind Alt+F4 to Close Window in the terminal keymap (#53091)

Closes #52774

## Summary
- Bind Windows `Alt+F4` to `workspace::CloseWindow` in the `Terminal`
keymap context
- Add a regression test covering the built-in Windows terminal keymap
entry

## Why
When the integrated terminal is focused, `Alt+F4` should close the
window instead of falling through to terminal keystroke handling.
Handling this in the Windows `Terminal` keymap keeps the fix aligned
with the rest of the terminal shortcut overrides.

## Validation
- `cargo test -p settings
windows_terminal_keymap_closes_window_on_alt_f4`

Release Notes:

- Fixed Alt+F4 on Windows so Zed closes even when the integrated
terminal is focused.

---------

Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>

assets/keymaps/default-windows.json

@@ -1226,6 +1226,7 @@
       "ctrl-delete": ["terminal::SendText", "\u001bd"],
       "ctrl-n": "workspace::NewTerminal",
       // Overrides for conflicting keybindings
+      "alt-f4": "workspace::CloseWindow",
       "ctrl-b": ["terminal::SendKeystroke", "ctrl-b"],
       "ctrl-c": ["terminal::SendKeystroke", "ctrl-c"],
       "ctrl-e": ["terminal::SendKeystroke", "ctrl-e"],