Xcode Updates

[josh]

This triennial commit brings the project up to speed with Xcode 15.3.

• With the new platform support features, we can now go from 4 targets (iOS+App, iOS+Extension, macOS+App, macOS+Extension) to just 2 targets (App, Extension). All the renames are just changes that affect the directory naming. Note, this doesn't make life any easier for submissions. The iOS and Mac App Store are still independent and require separate uploads.
• I did have to bump the minimum macOS requirement from 11 to 13. It has a new .windowResizability(.contentSize) which removes the need for all our hacks to make the macOS just a fixed non-resize window. f912bef
• Had to fiddle with some SwiftUI alignment issues I still don't quite understand. But it seems to be okay.

--
To: @dgraham
CC: @nakajima