[R] Implement decimal256()

[asfimport]

Reporter: Dragoș Moldovan-Grünfeld / @dragosmg
Assignee: Dragoș Moldovan-Grünfeld / @dragosmg

Related issues:

• [R] Implement decimal128() (to replace decimal()) (relates to)
• [R] Expand documentation for decimal() (is related to)

PRs and other links:

• GitHub Pull Request #11805
• GitHub Pull Request #11898
• GitHub Pull Request #11898

_{Note: This issue was originally created as ARROW-14844. Please see the migration documentation for further details.}

[asfimport]

Jonathan Keane / @jonkeane:
Issue resolved by pull request 11898
#11898