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ARROW-16038: [R] different behavior from dplyr when mutate's `.keep` option is set

This PR does two things to match some dplyr behaviour around column order:

1) Mimics dplyr implementation of `mutate(..., .keep = "none")` to append new columns after the existing columns (if suggested) as [per](https://github.com/tidyverse/dplyr/issues/6086)

2) As per this [discussion](https://github.com/tidyverse/dplyr/issues/6086), this required a bespoke approach to `transmute` as it not simply a wrapper for `mutate(..., .keep = "none")`. This cascades into needing to catch a couple edge cases.

I have also added some tests which will test for this behaviour.

Closes #12818 from boshek/mutate-keep

Authored-by: SAm Albers <sam.albers@gmail.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>