Available on macro
Once-only initialization.
See also:
https://aantron.github.io/luv/luv/Luv/Once
@:noExprstaticinit():Result<Once>
Allocates and initializes a once-only barrier.
@:noExprstaticonce(this:Once, callback:() ‑> Void):Void
Guards the given callback to be called only once.