feat: Confirmation before closing the tab

[billchurch]

via @israel-tsadok-silk from billchurch/webssh2#368

Optional confirmation before closing the tab. A very common keyboard shortcut for Linux is Ctrl-W, and people keep accidentally closing their ssh tabs when trying to delete a word. Browsers allow setting a beforeunload event listener that asks for a confirmation before the tab gets closed, which would be very helpful. When I tested this, I was able to add a confirmation but the session got disconnected anyway, so this might require some tweaking of the current event handlers.

[billchurch]

As far as I can tell, this should work for all browsers as long as they follow BeforeUnloadEvent, but I've only tested with Chrome 131.0.6778.86 and Safari 18.1.1 (20619.2.8.11.12) on macOS