fix(useDisplayMedia): stop stream when screen is not shared anymore

[robertrosman]

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Description

When the user stops sharing the screen, we need to stop the stream. Hence I've added an event listener to catch when the track ends, and call the stop function.

Additional context

Steps to reproduce the bug:

1. Open Google Chrome (on mac).
2. Start sharing screen (select a window, not just a tab).
3. Look for the small notification that says "vueuse.org is sharing a window" (1. in the attachment).
4. Click "Stop sharing".
5. Before this fix the stream turns black, but is still in enabled state (2. in the attachment). After this fix it is properly stopped.