Feature Request: Generate Xcode Previews friendly schemes

Description

We'd like to avoid using _CompileStub_.m as much as possible and display a proper source file list in Compile Sources to enable SwiftUI Previews.

Motivation

Since our product is composed of hundreds of static libraries, we would like to add a dedicated ios_framework for each module instead of building a huge ios_application for SwiftUI Previews.
However, as described here, those dynamic frameworks will contain only _CompileStub_.m if their primary module is included in the dependency graph of other top-level targets.

The current rules_xcodeproj's design is reasonable enough, but in a real world project, it's common for a module to be included in the dependency graph of multiple top-level targets like ios_unit_test and ios_application, so you may not able to use SwiftUI Previews naturally like native Xcode.

For now, we have solved this by generating Xcode projects focused on a specific ios_framework, but it would be great if rules_xcodeproj solves it in a comprehensive approach.

Start with the docs/faq.md section explaining _CompileStub_.m and compare it with the issue's SwiftUI Preview scenario involving ios_framework, ios_unit_test, and ios_application. Define a comprehensive approach for generating schemes with proper source lists, then verify that modules included by multiple top-level targets remain usable for SwiftUI Previews.