+str #16191 adds Flow.concat(Source):Flow method

Had some spare minutes to implemented this.
Let me know if this is the expected semantics you were hoping for.

This is "source things get fed through the flow op", the other one that would make sense is "once the flow is drained, we use the source, without passing through the flow".

Resolves #16191
oh sorry, typo on my end "through this Flow`