Enable Swift 6 support

Package@swift-6.0.swift

@@ -0,0 +1,33 @@
+// swift-tools-version:6.0
+
+import PackageDescription
+
+let package = Package(
+    name: "BSON",
+    platforms: [
+        .macOS(.v10_15),
+        .iOS(.v13)
+    ],
+    products: [
+        .library(
+            name: "BSON",
+            targets: ["BSON"])
+    ],
+    dependencies: [
+        .package(url: "https://github.com/apple/swift-nio.git", from: "2.46.0")
+    ],
+    targets: [
+        .target(
+            name: "BSON",
+            dependencies: [
+                .product(name: "NIOCore", package: "swift-nio"),
+            ],
+            swiftSettings: [
+                .enableExperimentalFeature("StrictConcurrency=complete"),
+            ]
+        ),
+        .testTarget(
+            name: "BSONTests",
+            dependencies: ["BSON"])
+    ]
+)

Tests/BSONTests/BSONPublicTests.swift

@@ -31,7 +31,7 @@ func assertInvalid(_ document: Document, file: StaticString = #filePath, line: U
     }
 }
 
-var binaryData: ByteBuffer = {
+let binaryData: ByteBuffer = {
     let alloc = ByteBufferAllocator()
     var buffer = alloc.buffer(capacity: 5)
     buffer.writeBytes([34,34,34,34,34])