`path_ext_set()` incorrectly removes `.` from extension

[mgirlich]

The intention was probably to remove a leading . instead of the first . encountered:

# incorrect
fs::path_ext_set("file", "csv.gz")
#> file.csvgz
# correct
fs::path_ext_set("file", ".csv.gz")
#> file.csv.gz

^{Created on 2023-04-19 with reprex v2.0.2}