Enable strict concurrency

Motivation:

To catch potential data races at build time.

Modifications:

- Enable strict concurrency in Package.swift. (No further modifications were made as no warnings/errors were thrown.)

Result:

Strict concurrency adoption.

Author: clintonpi

Package.swift

@@ -45,3 +45,9 @@ let package = Package(
         ),
     ]
 )
+
+for target in package.targets {
+    var settings = target.swiftSettings ?? []
+    settings.append(.enableExperimentalFeature("StrictConcurrency=complete"))
+    target.swiftSettings = settings
+}