Add eachrsplit iterator (#51646)
Unlike rsplit, this iterator returns split substrings right to left, but
other- wise it behaves just like eachsplit.
This design has been chosen to avoid either a costly double traversal of
the input string, or needing a stack to store the strings. Both of these
workarounds would lessen the appeal compared to simply using rsplit.
Closes https://github.com/JuliaLang/julia/issues/45385
