Enable MemberImportVisibility check on all targets (#19)

Enable MemberImportVisibility check on all targets. Use a standard
string header and footer to bracket the new block for ease of updating
in the future with scripts.

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Co-authored-by: George Barnett <gbarnett@apple.com>

Package.swift

@@ -43,6 +43,7 @@ let defaultSwiftSettings: [SwiftSetting] = [
   .swiftLanguageMode(.v6),
   .enableUpcomingFeature("ExistentialAny"),
   .enableUpcomingFeature("InternalImportsByDefault"),
+  .enableUpcomingFeature("MemberImportVisibility"),
 ]
 
 let targets: [Target] = [