feat: add `.duration` and `.time_ago` inline styles

[jjjermiah]

find myself frequently formatting elapsed times and relative timestamps when building clis and storing them in the user_themes option, it would be useful to support these out of the box.

• .duration : format seconds/timediff as human-readable duration
  i.e {.duration {elapsed}} -> [2m 3s] / [42s]
• .time_ago : format as relative time string
  i.e {.time_ago {ts}} -> "2 minutes ago"

both can take advantage of vendored formatters in the package R/time.R and R/time-ago.R without dependencies, and would be consistent with the .bytes feature from #740

[jjjermiah]

fyi: this might be a benign bug that doesnt affect anything

cli:::format_time$pretty_dt(as.difftime(90, units = 'secs'))
#> Error in `assert_diff_time()`:
#> ! could not find function "assert_diff_time"
cli:::format_time$pretty_dt(as.difftime(1.5, units = 'mins'))
#> Error in `assert_diff_time()`:
#> ! could not find function "assert_diff_time"

^{Created on 2026-04-22 with reprex v2.1.1}