cmFileAPI: Add "toolchains" object kind.

Fixes #19514

Source/cmFileAPIToolchains.h

@@ -0,0 +1,12 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include <cm3p/json/value.h>
+
+class cmFileAPI;
+
+extern Json::Value cmFileAPIToolchainsDump(cmFileAPI& fileAPI,
+                                           unsigned long version);