Support Coroutines Flow in Menu DSL

[DevSrSouza]

The Menu DSL should support Coroutines Flow.

For slots could be something like this:

val stateFlow: StateFlow<ItemStack>

slot(1, 5, null) {
  onRender {
     showingItem = stateFlow.value
  }
  onFlowChange(stateFlow) { value ->
    showingItem = value
    player.msg("Hi!")
  }
}