lc_time_tz_convert reform:
- enforce that only dates in the ISO format "%Y-%m-%d %H:%M:%S" are accepted
- apply clock api normalization to seemingly out of range time_values, as per
https://www.tcl.tk/man/tcl/TclCmd/clock.html#M25 - return the empty string every time input is invalid or a conversion cannot be determined
- return a value, as ISO date, only when conversion is successful
- prefer the clock api to perform the conversion, but fall back to the ref-timezones datamodel when the timezone is not supported by Tcl
