Add a Doxyfile and preliminary module-level docs.

src/util.js.h

@@ -1,5 +1,11 @@
 #pragma once
 
+/**
+** \defgroup util_js Utilities
+** This is becoming just a dumping ground of small helpers.
+** @{
+*/
+
 #include "quickjs.h"
 
 #include <stdbool.h>
@@ -28,3 +34,5 @@ const char* tf_util_function_to_string(void* function);
 		__typeof__(b) _b = (b); \
 		_a > _b ? _b : _a; \
 	})
+
+/** @} */