Wrong return type of Duration seconds function

**Describe the bug**
`seconds` method of Duration object specifies return type as number, but instead I get a string.
Type is specified here https://github.com/iamkun/dayjs/blob/dev/types/plugin/duration.d.ts#L38

`typeof dayjs.duration("PT30S").seconds()` gives `string`
**Expected behavior**
I expect it to either be a number or if the method should return a string, the type would be string as well.

**Information**
- Day.js Version - 1.10.4