Add limit to minimum window size (#13126)

Release Notes:

- Add a limit to the minimum window size on macOS.

Here's the minimum window before change:
<img width="121" alt="image"
src="https://github.com/zed-industries/zed/assets/38318044/9e907194-42e5-457e-91ea-96613426b479">

After change:
<img width="410" alt="image"
src="https://github.com/zed-industries/zed/assets/38318044/6e9c3057-9860-4f4b-9a73-c158ebac5ba9">

crates/gpui/examples/window_positioning.rs

@@ -51,6 +51,7 @@ fn main() {
                     kind: WindowKind::PopUp,
                     is_movable: false,
                     app_id: None,
+                    window_min_size: Size::default(),
                 }
             };