Add a `Windows_tz` module containing a CLDR-sourced mapping table from
Windows timezone names to canonical IANA names. Update
`timedesc_of_timestamp` to resolve Windows-style timezone identifiers
(e.g. `"W. Europe Standard Time"`) via this table before constructing a
`Timedesc.Time_zone.t`, falling back to the local timezone with a
warning if resolution fails entirely.
The `remind_sync` library was acting as a thin re-export layer. This
commit removes it entirely and moves the only non-trivial utility
(`show_error` for `Timedesc.Date.Ymd.error`) directly into
`bin/utils.ml`. Dead `[@@deriving show]` annotations on `rem`,
`week_first_day`, and `error` types are also removed.
