It's wrong for dayjs.tz when the parameter is string with utc offset.

**Describe the bug**

When this parameter is string without utc offset, it's right, keeping local time as same as moment.
`dayjs.tz("2023-04-19T04:00:00", 'Asia/Shanghai').format()`
output: 2023-04-19T04:00:00+08:00

But when this parameter is string with utc offset, it's wrong, neither keeping local time nor keeping the same time as same as moment.
`dayjs.tz("2023-04-19T04:00:00+04:00", 'Asia/Shanghai').format()`
output: 2023-04-19T00:00:00+08:00

**Expected behavior**

As same as moment, keeping the same time.
output: 2023-04-19T08:00:00+08:00

**Information**

- Day.js Version 1.11.9
- OS: [macOS]
- Browser [Chrome 116.0.5845.110]
- Time zone: [Asia/Shanghai]

Start by reproducing the reported dayjs.tz example with a string containing a UTC offset and compare it with the no-offset case and Moment's result. Trace the timezone parsing entry point and add a regression test covering the expected Asia/Shanghai output; done means the offset input preserves the expected local time.

Written by the indexing model from the issue text.