[CSM] Implement minetest.get_csm_restrictions()

fixes #8068

src/script/lua_api/l_client.h

@@ -96,6 +96,9 @@ class ModApiClient : public ModApiBase
 	// get_builtin_path()
 	static int l_get_builtin_path(lua_State *L);
 
+	// get_csm_restrictions()
+	static int l_get_csm_restrictions(lua_State *L);
+
 public:
 	static void Initialize(lua_State *L, int top);
 };