date: replace regex with hand-written format spec parser

Author: sylvestre

Cargo.lock

@@ -40,7 +40,6 @@ jiff = { workspace = true, features = [
   "tzdb-concatenated",
 ] }
 parse_datetime = { workspace = true }
-regex = { workspace = true }
 uucore = { workspace = true, features = ["parser", "i18n-datetime"] }
 
 [target.'cfg(unix)'.dependencies]