Setting identical timezone to the current timezone subtracts an hour

**Describe the bug**
In the timezone plugin, setting a timezone to a date can result in subtracting an hour when it wasn't supposed to be.

```
const today = dayjs('2021-10-10T23:00:00.000Z').tz('America/Los_Angeles') // converts to 4pm
const todayPacific = dayjs(today).tz('America/Los_Angeles') // converts to 3pm
```

The culprit is this line: https://github.com/iamkun/dayjs/blob/dev/src/plugin/timezone/index.js#L98

Doing `const target = date.toLocalString('en-US', { timeZone: timezone})` will return a string _without_ any timezone data so when the code executes the next line with `new Date(target)`, that date object is no longer in the correct timezone.

Recreated this behavior here:
https://codesandbox.io/s/damp-cherry-lzv2z?file=/src/App.js

I inspected the objects -- they both are set in the correct timezone for the date (`PDT`) with identical offset but their Unix timestamps are off. That shouldn't happen when setting timezone data.

**Expected behavior**
Setting an identical timezone to a date should not change the time.

**Information**
- Day.js Version 1.10.7
- OS: MacOS Big Sur
- Browser: Chrome 95
- Time zone: GMT-07:00 DST (Pacific Daylight Time)

Start in src/plugin/timezone/index.js at the linked line and reproduce the behavior with the issue's Day.js example or CodeSandbox. Trace the local-time conversion and verify that applying the same America/Los_Angeles timezone preserves the original Unix timestamp while retaining the correct PDT offset.

Written by the indexing model from the issue text.