Merge pull request #40 from SwiftPackageIndex/swift-6.0

Add Swift 6

Author: finestructure

Sources/SPIManifest/SwiftVersion.swift

@@ -20,6 +20,7 @@ public enum SwiftVersion: ShortVersion, Codable, CaseIterable {
     case v5_8 = "5.8"
     case v5_9 = "5.9"
     case v5_10 = "5.10"
+    case v6_0 = "6.0"
 
     public static var latestRelease: Self { .v5_10 }