Add API to cancel async jobs (#14602)

* Implement API to cancel async jobs

Co-authored-by: sfan5 <sfan5@live.de>

* update AsyncJob:cancel documentation from review

* Use IPC to unblock async

* review

* review async unblocking

* review

* Apply suggestions from code review

Co-authored-by: sfan5 <sfan5@live.de>

* minor licensing

---------

Co-authored-by: y5nw <y5nw@protonmail.com>
Co-authored-by: sfan5 <sfan5@live.de>

src/script/lua_api/l_async.h

@@ -0,0 +1,19 @@
+// Luanti
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#pragma once
+
+#include "lua_api/l_base.h"
+
+class ModApiAsync : public ModApiBase
+{
+public:
+	static void Initialize(lua_State *L, int top);
+private:
+	// do_async_callback(func, params, mod_origin)
+	static int l_do_async_callback(lua_State *L);
+	// cancel_async_callback(id)
+	static int l_cancel_async_callback(lua_State *L);
+	// get_async_threading_capacity()
+	static int l_get_async_threading_capacity(lua_State *L);
+};