[R] Implement .names argument inside across()

[asfimport]

ARROW-11699 adds support for dplyr::across inside a mutate(). The .names argument is not yet supported but should be added.

Reporter: Nicola Crane / @thisisnic

Related issues:

• [R] Implement dplyr::across() inside mutate() (is blocked by)

_{Note: This issue was originally created as ARROW-17364. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
This has now been implemented as part of ARROW-11699