feat: cbindgen configure file

src/to-json/cbindgen.toml

@@ -0,0 +1,3 @@
+language = "C"
+pragma_once = true
+include_version = true

src/to-json/to_json.h

@@ -1,3 +1,7 @@
+#pragma once
+
+/* Generated with cbindgen:0.23.0 */
+
 #include <stdarg.h>
 #include <stdbool.h>
 #include <stdint.h>