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/tests/test_Module.h

@@ -0,0 +1,100 @@
+/* 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/>.
+ */
+
+#include "lib/self_test.h"
+
+#include "ps/CLogger.h"
+#include "ps/Filesystem.h"
+#include "scriptinterface/ModuleLoader.h"
+#include "scriptinterface/ScriptInterface.h"
+
+class TestScriptModule : public CxxTest::TestSuite
+{
+public:
+	void setUp()
+	{
+		g_VFS = CreateVfs();
+		TS_ASSERT_OK(g_VFS->Mount(L"", DataDir() / "mods" / "_test.scriptinterface" / "module" / "",
+			VFS_MOUNT_MUST_EXIST));
+	}
+
+	void tearDown()
+	{
+		g_VFS.reset();
+	}
+
+	void test_StaticImport()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		TestLogger logger;
+		script.GetModuleLoader().LoadModule(rq, "include/entry.js");
+
+		// This test does not rely on export to the engine. So use the logger to check if it succeeded.
+		TS_ASSERT_STR_CONTAINS(logger.GetOutput(),"Test succeeded");
+	}
+
+	void test_Sequential()
+	{
+		{
+			ScriptInterface script{"Test", "Test", g_ScriptContext};
+			const ScriptRequest rq{script};
+			script.GetModuleLoader().LoadModule(rq, "empty.js");
+		}
+		{
+			ScriptInterface script{"Test", "Test", g_ScriptContext};
+			const ScriptRequest rq{script};
+			script.GetModuleLoader().LoadModule(rq, "empty.js");
+		}
+	}
+
+	void test_Stacked()
+	{
+		ScriptInterface scriptOuter{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rqOuter{scriptOuter};
+		{
+			ScriptInterface scriptInner{"Test", "Test", g_ScriptContext};
+			const ScriptRequest rqInner{scriptInner};
+			scriptInner.GetModuleLoader().LoadModule(rqInner, "empty.js");
+		}
+		scriptOuter.GetModuleLoader().LoadModule(rqOuter, "empty.js");
+	}
+
+	void test_ImportInFunction()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		TestLogger logger;
+		TS_ASSERT_THROWS(script.GetModuleLoader().LoadModule(rq, "import_inside_function.js"),
+			const std::invalid_argument&);
+		const std::string log{logger.GetOutput()};
+		TS_ASSERT_STR_CONTAINS(log, "import_inside_function.js line 3");
+		TS_ASSERT_STR_CONTAINS(log, "import declarations may only appear at top level of a module");
+	}
+
+	void test_NonExistent()
+	{
+		ScriptInterface script{"Test", "Test", g_ScriptContext};
+		const ScriptRequest rq{script};
+
+		const TestLogger _;
+		TS_ASSERT_THROWS(script.GetModuleLoader().LoadModule(rq, "nonexistent.js"),
+			const std::runtime_error&);
+	}
+};