gpui: Add layer to handle system permissions

[marcocondrache]

Right now, if a user tries to share their screen during a collab session without having previously granted permission, Zed just shows a message saying it can’t capture the screen and suggests checking system permissions. For users who aren’t comfortable digging around in system settings, this can be confusing and frustrating. It would be more helpful if Zed asked the system for permission directly and guided the user to the right place, which is how most apps handle this on macOS.

Having a permission layer like this will also be useful as accessibility support improves, since many macOS accessibility features require explicit permission before they can be used.

Release Notes:

• Zed now asks for screen capture permission when it hasn’t been granted yet and helps guide users to the correct system settings