Behavior question, Timezone. First parameter in tz() already wrote timezone? 關於時區的行為問題

Hi,

### Updated#1
I found out `dayjs.tz()` do it on purpose, for the reason. So `dayjs.tz()` doesn't support first parameter with timezone.
https://github.com/iamkun/dayjs/blob/9c20e77caf7b1b5eccf418175203b198d4e29535/src/plugin/timezone/index.js#L3
```js
const typeToPos = {
year: 0,
month: 1,
day: 2,
hour: 3,
minute: 4,
second: 5
}
```

### Q1
If first parameter have been added -0400, matched timezone Etc/GMT+4
should function `fixOffset` be called?

```js
// notice the hour part

// situation 1 output: 2021-03-31T04:00:00-04:00
dayjs.tz('2021-03-31T00:00:00-0400', 'Etc/GMT+4').format();

// situation 2 output: 2021-03-31T00:00:00-04:00
dayjs.tz('2021-03-31T00:00:00', 'Etc/GMT+4').format();
```

In moment,
I've add -0400 in moment.tz, it will check needsOffset(), which return false for m._tzm is -240.

https://github.com/moment/moment-timezone/blob/ffc41900fed9efa9a31e87f0eadd66ec86b5046a/moment-timezone.js#L564
```js
// situation 3 output: 2021-03-31T00:00:00-04:00
moment.tz('2021-03-31T00:00:00-0400', 'Etc/GMT+4').format();

// situation 4 output: 2021-03-31T00:00:00-04:00
moment.tz('2021-03-31T00:00:00', 'Etc/GMT+4').format();
```

### Q2
And there is one other situation, time with different timezone.
```js
dayjs.tz('2021-03-31T00:00:00+0800', 'Etc/GMT+4').format(); // "2021-03-30T16:00:00-04:00"

moment.tz('2021-03-31T00:00:00+0800', 'Etc/GMT+4').format(); // "2021-03-30T12:00:00-04:00"
```

### question is
How would it be designed in dayjs?

=====
你好，

假設系統是北京時間。

在第一個情況可以看到因為多寫了 -0400，因此`new Date()`解析後，把時區差也給算上了，但後續的`fixOffset`為了補上時間差，反而造成了跟 moment 不一樣的結果，因為 moment 有多一層確認。

在第二種情況，多寫了北京時區+0800，tz 第二個參數帶入美東時區，亦跟 moment 不一樣

並不是說全然跟 moment 一致就是對的

只是好奇的是，這是故意被設計成這樣的嗎，或是其中有什麼概念在裡頭，想了解。

感謝

Start with src/plugin/timezone/index.js at the linked typeToPos and fixOffset logic, then compare the four Day.js and Moment timezone examples in the issue. Determine the intended behavior for explicit offsets, matching offsets, and different timezones; done requires a clear design decision for these cases, supported by the project’s timezone behavior.

Written by the indexing model from the issue text.