Adds a `WindowVisibility` API to GPUI: `Window::visibility()` /
`Window::is_visible()`, `Window::observe_window_visibility`, and
`Context::observe_window_visibility`, backed by one platform signal per
backend — `NSWindow.occlusionState` on macOS, `WS_VISIBLE` plus the
minimized state on Windows, the xdg-shell v6 `suspended` toplevel state
on Wayland, mapped state plus `VisibilityNotify` on X11, and the Page
Visibility API on web. Only transitions are reported, the initial value
is read from the platform when the window is created, and the two
`PlatformWindow` methods have no default so a backend cannot silently
opt out. The per-platform limits (Windows can't see a fully covered
window, pre-v6 Wayland compositors never report `Hidden`) are documented
on the type.

It also adds `Platform::on_system_sleep` / `App::on_system_sleep`,
mirroring the existing wake hook, on macOS
(`NSWorkspaceWillSleepNotification`), Windows (`PBT_APMSUSPEND`), and
Linux (logind `PrepareForSleep(true)`, which the listener previously
discarded). Nothing in Zed subscribes to either yet; the hang-telemetry
journal will use visibility to stop measuring frames that could not have
been presented, and the sleep/wake pair to bracket time the process was
not running.

Release Notes:

- N/A