Support ISO 8601 strings in malli.experimental.time/schemas

[devurandom]

malli.experimental.time/schemas already supports java.time objects, but it would be convenient if there was a pre-defined set of ISO 8601 string schemas, so I could validate "2025-10-31T14:28:Z" without having to transform it to a java.time object first.

Currently, I can either try to transform the string to a java.time.*, and get unhelpful messages in case of validation errors (cf. #1114), or I can validate using my own hand-crafted regexp:

(ns repro
  (:require
    [malli.core :as m]
    [malli.error :as me]))

(def offset-date-time-string
  "Malli schema for ISO 8601 extended format date time"
  [:re {:error/message "should be an ISO 8601 extended format date time"}
   #"^\d{4}-[01]\d-[0-3]\dT[0-2]\d:[0-5]\d:[0-5]\d(\.\d\d\d)?([+-][0-2]\d:[0-5]\d|Z)$"])

(me/humanize (m/explain offset-date-time-string "2025-10-31T14:28:Z"))
;=> ["should be an ISO 8601 extended format date time"]

Adding string schemas might already be a first step towards #60, too.

See-also: #1114
See-also: #60

[opqdonut]

That sounds like a good addition. PR welcome!