sum() method for Duration class

[d-morrison]

re: #902

I'm trying to preserve the Duration class through sum(); for example:

Before:

x = lubridate::duration(1:2)
sum(x)
#> [1] 3

After:

x = lubridate::duration(1:2)
sum(x)
#> [1] "3s"
class(x)
#> [1] "Duration"
#> attr(,"package")
#> [1] "lubridate"

^{Created on 2025-01-15 with reprex v2.1.1}

Not sure if this will be helpful, but figured it was worth trying; glad to keep working on a better solution based on your feedback.
I used purrr::reduce(), which I'm guessing introduces an unwanted dependency.
I tried base::Reduce() but it doesn't preserves the Duration class unfortunately.

[vspinu]

This is really nice addition but I would prefer to avoid adding extra dependency just for this change. It's straightforward to do in plain R.