[tz] Add new timezone for Hanoi Capital, Vietnam

[Paul Eggert]

KP wrote:
> - After merging with Ha Tay, Hanoi (capital) is the largest province in
> Vietnam

tzdb goes by cities, not provinces. citypopulation.de estimates a population for 
Hanoi (including Ha Tay) that is less than that of Ho Chi Minh City. 
https://www.citypopulation.de/Vietnam-Cities.html gives 2017 estimates of 7.4 
million for the former, and 8.4 million for the latter. Although both are 
growing, the ratio of Ho Chi Minh City vs Hanoi (including Ha Tay) population 
also appears to be growing, suggesting that Hanoi is not catching up.

Even if Hanoi became somewhat larger than Ho Chi Minh City, the following 
guideline in <https://data.iana.org/time-zones/theory.html> would still apply:

"Do not change established names if they only marginally violate the above 
guidelines. For example, do not change the existing name Europe/Rome to 
Europe/Milan merely because Milan's population has grown to be somewhat greater 
than Rome's."

> - +7:00 (North Vietnam and All Vietnam since 1975) timezone has been the
> longest unified timezone ever

I'm not sure what's meant by "longest unified timezone ever". In tzdb, the 
location that has been at its current UT offset for the longest time is 
Atlantic/South_Georgia, unchanged since 1890. In contrast, tzdb's sources 
indicate that Hanoi changed UTC offset in 1954, which makes Hanoi less stable, 
time-wise, than Bangkok (last change 1920) or Lagos (last change 1919). At any 
rate, longevity of this sort has not been part of the criteria we've used to 
choose Zone names.