unused_declaration does not catch View only used in PreviewPeovider

[iressler]

New Issue Checklist

• [x] Updated SwiftLint to the latest version
• [x] I searched for existing GitHub issues

Describe the bug

When a View is only used by it's matching PreviewProvider unused_declarations does not report it as unused. E.g. if this is in a file and the View is not referenced anywhere else SwiftLint will not report any issues:

struct UnusedView: View {
  var body: some View {
    Text("Hello, World!")
  }
}

struct UnusedView_Previews: PreviewProvider {
  static var previews: some View {
    UnusedView()
  }
}

This may be expected behavior, the View is being used, however I was surprised by it and don't consider that a "real" use. If this isn't reported as unused then no SwiftUI View with a Preview will ever be "unused", even though they aren't actually being used anywhere.

Complete output when running SwiftLint, including the stack trace and command used

$ swiftlint analyze --compiler-log-path <path>
Analyzing Swift files in current working directory
Collecting 'UnusedDeclarationsPreviewsApp.swift' (1/3)
Collecting 'ContentView.swift' (2/3)
Collecting 'UnusedView.swift' (3/3)
Analyzing 'UnusedDeclarationsPreviewsApp.swift' (1/3)
Analyzing 'ContentView.swift' (2/3)
Analyzing 'UnusedView.swift' (3/3)
Done analyzing! Found 0 violations, 0 serious in 3 files.

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.50.3
• Installation method used (Homebrew, CocoaPods, building from source, etc)? HomeBrew
• Paste your configuration file:

analyzer_rules:
  - unused_declaration

• Are you using nested configurations? No
• Which Xcode version are you using (check xcodebuild -version)? Xcode 14.2 Build version 14C18
• Do you have a sample that shows the issue? See above for code, but I've also provided a sample project.

[SimplyDanny]

Seems to be a good idea to add an option to the rule that makes it treat types only used in previews as unused.