Feature Request: `xcschemes.scheme` should have an `adjust_xcode_previews` attribute

[brentleyjones]

xcodeproj has the adjust_schemes_for_swiftui_previews attribute (which should be renamed), which globally applies a change to all schemes. It can be desirable to adjust that behavior on a scheme-by-scheme basis though. So xcschemes.scheme should have an adjust_xcode_previews attribute which allows overriding the global value.

[brentleyjones]

This is no longer needed, since we no longer adjust the scheme in incremental generation mode.