Add a Window method to center the window

[danielvschoor]

Is your feature request related to a problem? Please describe.
My splash screen is not centered when my app launches. I’ve worked around it for now by hardcoding the position to the center of a 1920x1080 display. This will not be centered on 4K or other displays

Describe the solution you'd like
A method on the Window struct called ‘set_position_center()’ or ‘center()’ that centres the window based on the the actual screen size and the window size.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

I’m just creating this issue for tracking, I might even give this feature a go myself when I get some spare time

[nothingismagick]

Thanks @danielvschoor - technically this issue should probably be resolved at the https://github.com/tauri-apps/wry layer - but I will let @wusyong decide how he wants to handle that.