[R] Update parse_date_time to accept a string with no separators

[asfimport]

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

Related issues:

• [R] lubridate functions for 9.0.0 (is a child of)

PRs and other links:

• GitHub Pull Request #13196

_{Note: This issue was originally created as ARROW-16446. Please see the migration documentation for further details.}