DayJS Timezone Does Not Accept UTC (Zulu) Timestamp as Argument

**Describe the bug**
Hey! Just first things first I want to say thanks for the library. Our team is in the process of getting rid of momentJS across the whole app and we are attempting to convert fully over to dayJS but ran into this issue today.

In Moment-Timezone you can pass a timestamp as follows using the Zulu format and an olson timezone to convert a date.

```
moment.tz("2020-12-14 19:58:00.000Z", 'America/New_York').format()
// returns 2020-12-14T14:58:00-05:00
```

DayJS appears to be ignoring the "Z" formatting:

```
dayjs.tz('2020-12-14 19:58:00.000Z', 'America/New_York').format()
// returns 2020-12-14T19:58:00-05:00
```

**Expected behavior**
I would expect the output of dayJS and MomentJS this to be the same given the same input. There might be a DayJS Method we could use to get the same result but I am currently unaware of it.

**Information**
- Day.js Version: 1.9.7
- OS: Mac OS Mojave 10.14.6
- Browser [e.g. chrome 62]
- Time zone: US Eastern Standard Time (EST -5:00)

Start by reproducing the reported dayjs.tz(...) call with the Zulu timestamp and America/New_York timezone, then trace the timezone parsing and formatting entry points. Done means the call preserves the UTC offset and returns the expected 2020-12-14T14:58:00-05:00 result, matching the Moment-Timezone example.

Written by the indexing model from the issue text.