Change leading characters in label_date_short()

[teunbrand]

This PR aims to fix #442.

Briefly, adds an additional argument label_date_short() that swaps out leading zeroes with other characters.
Example:

devtools::load_all("~/packages/scales/")
#> ℹ Loading scales
x <- seq(Sys.Date(), Sys.Date() + 365, by = "2 months")
labels <- label_date_short(format = c("%Y", "%m", "%d", "%H:%M"), leading = "")
labels(x)
#> [1] "7\n10\n2024" "7\n12"       "7\n2\n2025"  "7\n4"        "7\n6"       
#> [6] "7\n8"        "7\n10"

^{Created on 2024-10-07 with reprex v2.1.1}