Fix transform_time() because of lubridate incompatibility

[ndevln]

Hi,

this change is really minor, but fixes a strange incompatibility with lubridate.

The bug:

library(scales)
library(lubridate)

fun <- transform_time("UTC")
fun$inverse(as_datetime("2024-01-05")) + days(1)
#> Error: Incompatible classes: <POSIXt> + <Period>

This bug occurs in the wild, since scales::transform_time() is the default transformation for ggplot2::scale_x_datetime().

The underlying problem is that lubridate only provides a method for POSIXct and not for POSIXt. And the order of the classes is therefore important.
https://github.com/tidyverse/lubridate/blob/fea9692686dcb0a968fa068d7f8dfdb41bbea581/R/ops-addition.r#L129

Also, the new order of the classes is the default order of as.POSIXct:

class(as.POSIXct("2024-01-05"))
#> [1] "POSIXct" "POSIXt" 

Fixing this problem in scales seemed the easiest, since the alternative would be adding a lot of new methods to lubridate.

I don't know if there is a reason for the inverse class order. But I hope this is an easy fix.

Greetings
Alex