Support Swift 6 mode (#65)

* Enable swift 6 mode

* Attach Sendable

* Stop async context because enumerator can use only synchronous contexts

* Fix global shared mutable state for MustacheContentTypes

* Revert "Fix global shared mutable state for MustacheContentTypes"

This reverts commit d4ccc83e07aeb48f4aa4024b71eb8e5f70131bc5.

* Use  instead of lock

* Support 5 and 6 versions

* Lock on access in Swift 6

* Support 5.9

* Revert "Support 5.9"

This reverts commit 9845b3bc448b2af7238c3ac88aabe6d764b2e667.

* Fix 5.9 compatibility

* Unify to manage the same lock logic in 5.9 and 6

* Add withLock backport in NSLock

Sources/Mustache/NSLock+Backport.swift

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Hummingbird server framework project
+//
+// Copyright (c) 2021-2021 the Hummingbird authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See hummingbird/CONTRIBUTORS.txt for the list of Hummingbird authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+#if compiler(<6.0)
+import Foundation
+
+extension NSLock {
+    func withLock<Value>(_ operation: () throws -> Value) rethrows -> Value {
+        self.lock()
+        defer {
+            self.unlock()
+        }
+        return try operation()
+    }
+}
+#endif