[positioner] Adjust Window Position Offsets

When using the `Position::TrayCenter` enum, I've noticed that the window opens extremely close to the system tray — there's practically no gap at all. For aesthetic and usability purposes, this is less than ideal.

I believe there isn't a way to adjust the position offsets, which would allow for a more customizable experience based on user or application needs. Thus, I would like to request the feature of adjustable window position offsets for the tauri-plugin-positioner. This could potentially be a configuration option that allows us to define additional pixel offsets (x and y) from the position provided by the `Position::` enum.