Conditional Relative Time Locale Update

Hello, is there a way to conditionally update relativeTime?

The current way we do it from docs is like this:

```js
dayjs.extend(updateLocale)

dayjs.updateLocale('en', {
relativeTime: {
future: "in %s",
past: "%s ago",
s: 'a few seconds',
m: "a minute",
mm: "%d minutes",
h: "an hour",
hh: "%d hours",
d: "a day",
dd: "%d days",
M: "a month",
MM: "%d months",
y: "a year",
yy: "%d years"
}
})
```

In my code, I want to keep this formats but also want to update it conditionally?

I have some code that I want to use this format

```
dayjs('2021-09-29').fromNow() // 1 day ago
```

and I also have some code that I want to update some if a condition is met somewhere

```
dayjs.updateLocale('en', {
relativeTime: {
...
d: "1d",
dd: "%dd",
...
}
})
dayjs('2021-09-29').fromNow() // 1d
```

Start with the documented updateLocale API and the relativeTime/fromNow examples shown in the issue. Determine how conditional selection between the existing wording and abbreviated wording should work, then verify that both example calls produce the intended output without changing the default format.

Written by the indexing model from the issue text.