Update UIKitBackend Picker implementation to match .menu style

[stackotter]

Currently UIKitBackend's Picker implementation produces pickers that look like so:

This style of picker doesn't act anything like the pickers rendered by other backends and makes it tricky to build layouts that work on both desktop platforms and iOS. For this reason, I believe that we should match our picker style to SwiftUI's default .menu picker style:

See Sarunw's picker style article to see what the other available picker styles look like.

The current UIKitBackend Picker implementation uses UIPickerView which is explicitly only for wheel-style pickers. We'll probably have to try to replicate SwiftUI's .menu picker style with something similar to our Menu button implementation.