added isDST and getAbbreviation to TimeZoneTime

Hi, Just found this lib and I love it.

I am a timezone-js refugee, I hope I can find asylum here.

Wanted to add a couple of methods that I need.
Let me know if this contribution should be done in a different way; I will gladly update it.
My editor ate some trailing whitespace (LMK if that is a problem), I would recommend adding a lint rule to forbid them.

Also, what is the point of ```WallTimeDate```? It seems like an unused (and wrong, see line 103) duplicate of ```TimeZoneTime```, should it be removed?

Best regards,
Vadim