+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
I don't understand this sentence. Which is "that source"?