Add function get current time zone for Windows

[darrenldl]

Reference input_tz_file from Jane Street's Core_kernel.Zone for implementation details for handling in Linux case: https://github.com/janestreet/core_kernel/blob/master/src/zone.ml

Exact approach required, from Python community: https://github.com/regebro/tzlocal/tree/master/tzlocal

• The windows registry access stuff could possibly be replicated via batch command: https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/reg-query

[darrenldl]

Currently windows support is still missing, but not exactly what approach is best.

[darrenldl]

Superseded by #48