Support JavaScript modules

- With modules JavaScript code can be split up into multiple files. We
	already implemented such a mechanism (`Engine.LoadLibrary`) in
	multiple parts of the engine. The advantage of using modules is
	that it's standart (JS-devs are familiar with it) and it doesn't
	has to be implemented multiple times.
	Note that `Engine.LoadLibrary` loads all files in a directory
	while the new `import` only loads one file.

- With modules seemingly global variables are local to that
	script/module. We already implemented such a mechanism
	(`ScriptInterface::LoadScript`).

source/scriptinterface/ModuleLoader.h

@@ -0,0 +1,62 @@
+/* Copyright (C) 2025 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef INCLUDED_SCRIPTMODULELOADER
+#define INCLUDED_SCRIPTMODULELOADER
+
+#include "lib/file/vfs/vfs_path.h"
+#include "scriptinterface/ScriptTypes.h"
+
+#include <unordered_map>
+
+class ScriptContext;
+class ScriptRequest;
+
+namespace Script
+{
+class ModuleLoader
+{
+public:
+	friend ScriptContext;
+
+	class CompiledModule
+	{
+	public:
+		CompiledModule(const ScriptRequest& rq, const VfsPath& filePath);
+		JS::PersistentRootedObject m_ModuleObject;
+	};
+
+	using RegistryType = std::unordered_map<VfsPath, CompiledModule>;
+
+	/**
+	 * Load the specified module and all module it imports recursively.
+	 *
+	 * @param rq @c globalThis is taken from this @c ScriptRequest.
+	 * @param modulePath The path to the file which should be loaded as a module.
+	 */
+	void LoadModule(const ScriptRequest& rq, const VfsPath& modulePath);
+
+private:
+	// Functions used by the `ScriptContext`.
+	[[nodiscard]] static JSObject* ResolveHook(JSContext* cx, JS::HandleValue,
+		JS::HandleObject moduleRequest) noexcept;
+
+	RegistryType m_Registry;
+};
+} // namespace Script
+
+#endif // INCLUDED_SCRIPTMODULELOADER