Parse with a default offset (if string does not specify)

I'm working with a service that provides two types of date strings:

- local times, which need to be parsed in the context of the local offset / tz
- iso times, which specify their offset (in my case, always UTC)

I've resorted to this hacky parse helper:

```js
function parseTime(s, tz) {
if (s.endsWith('Z')) return dayjs(s).tz(tz)
return dayjs.tz(s, tz)
```

This is needed because dayjs.tz overrides the offset, even for iso strings which specify offset.

It feels like a bug in dayjs.tz, and in fact had different behavior in 1.9.7...

Dayjs 1.10.7

Start by reproducing the reported behavior with dayjs.tz on offset-less and UTC-suffixed strings using Day.js 1.10.7, then compare it with the behavior in 1.9.7. Trace how dayjs.tz handles an offset already present in the input; done means specified offsets are preserved while offset-less strings use the supplied timezone.

Written by the indexing model from the issue text.