fix: handle Dayjs object input in dayjs.utc()

[Bahtya]

Problem

When passing a Dayjs object to dayjs.utc(), the result is incorrect — it returns today's date instead of the passed date.

const d = dayjs(2018-09-06)
dayjs.utc(d).format() // expected: 2018-09-06T00:00:00Z, actual: wrong date

This happens because dayjs.utc() passes the raw Dayjs object directly to the config, which goes through parseDate() → new Date(date), producing incorrect results.

Solution

Add an isDayjs check at the top of dayjs.utc() to convert to a native Date first:

dayjs.utc = function (date) {
    if (dayjs.isDayjs(date)) date = date.toDate()
    ...
}

This mirrors how the core dayjs() function already handles Dayjs input.

Fixes #1241