Add .asFlow() extensions for ObservableValue to kotlinx-coroutines-javafx library

[qwwdfsad]

It would be nice if all observable properties could be easily converted to Flow when coding with TornadoFX. This could be added either for javafx.beans.value.ObservableValue or (maybe even better) for the entire javafx.beans.Observable interface if possible.

Originally reported here: https://youtrack.jetbrains.com/issue/KT-35389

[LittleLightCz]

Thank you, I am watching like a hawk 🦅 ☺️