Add on_new_window setting (#63522)

> [!NOTE]
> This is a remake of #63335 as it's unlikely to be reopened just from
my
[mention](https://github.com/zed-industries/zed/pull/63335#issuecomment-5452544138).
Apologies for the noise!

# Objective

Closes #59368, where users (including me) are asking for a way to show
the Launchpad instead of an empty tab when opening a new window. As per
advice from
https://github.com/zed-industries/zed/issues/60720#issuecomment-4936162420,
adding a separate setting from `restore_on_startup` is more ideal.

`on_new_window` allows for two values:
- `empty_tab`
- `launchpad` (default)

## Testing

Added a regression test which opens a new window with `on_new_window`
set to `launchpad` and asserts that no item gets opened in the window's
pane. I've verified it fails without the setting gate, and confirmed
configuring this setting works manually (see Showcase).

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase


https://github.com/user-attachments/assets/bf339bd1-8c62-4243-ac9d-1045d1a14865

---

Release Notes:

- Added `on_new_window` setting to configure the opening of new windows.

assets/settings/default.json

@@ -210,6 +210,13 @@
   //  3. Never close the window
   //         "when_closing_with_no_tabs": "keep_window_open",
   "when_closing_with_no_tabs": "platform_default",
+  // What to show when opening a new window.
+  // May take 2 values:
+  //  1. Show an empty untitled buffer
+  //         "on_new_window": "empty_tab"
+  //  2. Show the launchpad with recent projects
+  //         "on_new_window": "launchpad"
+  "on_new_window": "launchpad",
   // What to do when the last window is closed.
   // May take 2 values:
   //  1. Use the current platform's convention