[tz] Add new timezone for Hanoi Capital, Vietnam

[Clive D.W. Feather]

Tim Parenti said:
>> Also pay attention that, because it is the time history of Ho Chi Minh
>> city not time history of South Vietnam, the rule represent the transition
>> of clock due to change in political status of the city as the city was
>> being captured by Northern Vietnam.
> Another very good point ??? and one that's easy to miss! ??? which should
> probably also be added to the commentary in the file.

Hmm, do we need another zone for that? Or will there be a string of such
zones as the border moved south? I don't know the history of Vietnam well
enough to know.

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