[R] Add support for locale-specific day of week (and month of year?) returns from timestamp accessor functions

The R binding for the wday date accessor added in this PR currently doesn't support returning the string representation of the day of the week (e.g. "Mon") and only supports the numeric representation (e.g. 1).

We should implement this, though discussion should be had about whether this belongs at the R or C++ level.

**Reporter**: [Nicola Crane](https://issues.apache.org/jira/browse/ARROW-13133) / @thisisnic
#### Related issues:
- [[R] strftime bindings](https://github.com/apache/arrow/issues/29303) (is blocked by)

_{**Note**: *This issue was originally created as [ARROW-13133](https://issues.apache.org/jira/browse/ARROW-13133). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the R binding for the wday date accessor introduced in pull request #10507, and compare its numeric return behavior with the requested locale-specific day and month strings. Resolve whether the change belongs in the R binding or at the C++ level; done means the accessor can return localized names rather than only numeric values.

Written by the indexing model from the issue text.