+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 find that still confusing :) Is "this Flow" the receiver of the call or the result? And in which way are the elements passed through it? Do we need this sentence at all?

Wouldn't this be enough:
"Returns a new flow that concatenates a secondary source to this flow so that the first element emitted by the given source is emitted after the last element of this flow."