[R] Implement use of `where()` inside `across()

[asfimport]

Reporter: Nicola Crane / @thisisnic
Assignee: Nicola Crane / @thisisnic

Related issues:

• [R] Replace vars_select, vars_rename with eval_select, eval_rename (relates to)
• [R] Support tidyselect where() selection helper in dplyr verbs (relates to)
• [R] Implement dplyr::across() inside mutate() (is related to)

_{Note: This issue was originally created as ARROW-17698. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
This was resolve in ARROW-12105