ENH: add abiility for json.dumps to parse and add timezones
- [x] closes #12997
- [x] tests modofied
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Ok, so this is a doozy AND it's not done, but it can be merged as is.
What do we do:
We add timezones to json output!
Before:
```
ts = pd.Timestamp('2020-10-5 4:00:05', tz='US/Pacific')
print(pd.io.json.dumps(ts, iso_dates=True))
print(ts.isoformat())
```
would provide
`"2020-10-05T11:00:05.000Z"`
and
`"2020-10-05T04:00:05.000-07:00"`
respectively.

Now they both produce
``"2020-10-05T04:00:05.000-07:00"``

So what do I mean by not complete?
This still doesn't work for arrays that have datetime values. This only works for single datetime values.
So:
`pd.DataFrame({'foo': range(5)}, index=pd.date_range('2020-10-12', freq='H', periods=5, tz='US/Eastern')).to_json(date_format='iso')`

will still produce:
`'{"foo":{"2020-10-12T04:00:00.000Z":0,"2020-10-12T05:00:00.000Z":1,"2020-10-12T06:00:00.000Z":2,"2020-10-12T07:00:00.000Z":3,"2020-10-12T08:00:00.000Z":4}}'`

and not 
`'{"foo":{"2020-10-12T00:00:00.000-04:00":0,"2020-10-12T01:00:00.000-04:00":1,"2020-10-12T02:00:00.000-04:00":2,"2020-10-12T03:00:00.000-04:00":3,"2020-10-12T04:00:00.000-04:00":4}}'`

Working on it currently. Open to recommendations!