API: str.cat will align on Series
Fixes issue #18657, fixed existing tests, added new test; all pass.

After I pushed everything and thought about it some more, I realised that one may argue about the default alignment-behavior, and whether it should be changed to `join=outer`. The behavior as implemented is compatible with the current requirement that everything be of the same length. To me, it is more intuitive that the concatenated `other` is added to the current series without enlarging it, but I can also see the argument why that restriction is unnecessary.

PS. This is my first PR, tried to follow all the rules. Sorry if I overlooked something.

Edit: Also fixes #20842 