Duration validation tooling

Could it be possible to expose a method to validate a duration like the isValid method on the Dayjs date object?
```
console.log(dayjs.duration("PT1M").isValid()) // true
console.log(dayjs.duration("randomstring").isValid()) // false
// etc.
```
In the meantime, I will be using the inner regex to validate my durations, but having an embedded validation method would be a big plus.

edit: using isNaN(duration.milliseconds()) seems to be easier to use instead of regex.

Start at the dayjs.duration entry point and review how duration strings are currently validated, including the inner regex and milliseconds() behavior mentioned in the issue. Define the expected isValid() result for valid and invalid duration examples, then add coverage for those cases and confirm the public duration API exposes the method.

Written by the indexing model from the issue text.