Add: [Script] GSAsyncMode to set async mode of gamescript commands (#10913)

In asynchronous mode, don't wait for result of executed command,
just fire-and-forget, and return estimated cost/result

src/script/api/script_asyncmode.hpp

@@ -0,0 +1,63 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD 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, version 2.
+ * OpenTTD 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 OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file script_asyncmode.hpp Switch the script instance to Async Mode. */
+
+#ifndef SCRIPT_ASYNCMODE_HPP
+#define SCRIPT_ASYNCMODE_HPP
+
+#include "script_object.hpp"
+
+/**
+ * Class to switch current mode to Async Mode.
+ * If you create an instance of this class, the mode will be switched to
+ *   either Asynchronous or Non-Asynchronous mode.
+ *   The original mode is stored and recovered from when ever the instance is destroyed.
+ * In Asynchronous mode all the commands you execute are queued for later execution. The
+ *   system checks if it would be able to execute your requests, and returns what
+ *   the cost would be. The actual cost and whether the command succeeded when the command
+ *   is eventually executed may differ from what was reported to the script.
+ * @api game
+ */
+class ScriptAsyncMode : public ScriptObject {
+private:
+	ScriptAsyncModeProc *last_mode; ///< The previous mode we were in.
+	ScriptObject *last_instance;    ///< The previous instance of the mode.
+
+protected:
+	static bool AsyncModeProc();
+	static bool NonAsyncModeProc();
+
+public:
+#ifndef DOXYGEN_API
+	/**
+	 * The constructor wrapper from Squirrel.
+	 */
+	ScriptAsyncMode(HSQUIRRELVM vm);
+#else
+	/**
+	 * Creating instance of this class switches the build mode to Asynchronous or Non-Asynchronous (normal).
+	 * @note When the instance is destroyed, it restores the mode that was
+	 *   current when the instance was created!
+	 * @param asynchronous Whether the new mode should be Asynchronous, if true, or Non-Asynchronous, if false.
+	 */
+	ScriptAsyncMode(bool asynchronous);
+#endif /* DOXYGEN_API */
+
+	/**
+	 * Destroying this instance resets the asynchronous mode to the mode it was
+	 *   in when the instance was created.
+	 */
+	~ScriptAsyncMode();
+
+	/**
+	 * @api -all
+	 */
+	virtual void FinalRelease();
+};
+
+#endif /* SCRIPT_ASYNCMODE_HPP */