Add development support for reloading templates when you render them (#30)

* Add support for reloading templates when you render them

* comment

* Ensure reload is only available in DEBUG

* move preprocessor block

* swift format

* MustacheTemplate.init?(filename:) internal

* Only pass reload flag down in DEBUG builds

* Rebase with main

Sources/Mustache/Template+FileSystem.swift

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Hummingbird server framework project
+//
+// Copyright (c) 2021-2024 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
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+extension MustacheTemplate {
+    /// Internal function to load a template from a file
+    /// - Parameters
+    ///   - string: Template text
+    ///   - filename: File template was loaded from
+    /// - Throws: MustacheTemplate.Error
+    init?(filename: String) throws {
+        let fs = FileManager()
+        guard let data = fs.contents(atPath: filename) else { return nil }
+        let string = String(decoding: data, as: Unicode.UTF8.self)
+        self.tokens = try Self.parse(string)
+        self.filename = filename
+    }
+}