Use Swift 5 language mode

`MachPort` is fine with Swift 6.1 and up, but does not compile with Swift 6.0’s Swift 6 language mode.
`import CSystem` is a problem in the Linux version (inconsistently imported as implementation-only.)
The Wasm implementation seems not concurrency-safe.

Author: glessard

Package.swift

@@ -139,5 +139,6 @@ let package = Package(
       exclude: testsToExclude,
       cSettings: cSettings,
       swiftSettings: swiftSettings),
-  ])
-
+  ],
+  swiftLanguageVersions: [.v5]
+)