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

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.7
+// swift-tools-version:5.9
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
@@ -13,5 +13,6 @@ let package = Package(
     targets: [
         .target(name: "Mustache", dependencies: []),
         .testTarget(name: "MustacheTests", dependencies: ["Mustache"]),
-    ]
+    ],
+    swiftLanguageVersions: [.v5, .version("6")]
 )