Duration with ISO8601 months

> However, I'm not sure if we should count 2 months as 60 days or 61 days. You can tell from the unit test code I added.

_Originally posted by @iamkun in https://github.com/iamkun/dayjs/issues/949#issuecomment-652810292_

---

As already mentioned by @iamkun, there is a problem calculating durations for months relative to a date.

For example, as `P2M` (two months) can be either:
* 59 (28 days february + 31 days march 2023)
* 60 days (31 days in january and 29 days in february 2024)
* 61 (most cases)
* 62 (31 days in july and august 2021)

This leads to faulty results, as [demonstrated here](https://stackblitz.com/edit/dayjs-playground-iwkmc2?file=index.js).

---

So using the input `2020-04-07T09:00:00` and adding two months using the ISO 8601 duration `P2M`:

```ts
dayjs('2020-04-07T09:00:00').add(dayjs.duration('P2M')).format();
```
results in `2020-06-06T09:00:00+02:00`.

And

```ts
dayjs('2020-04-07T09:00:00').add(dayjs.duration({ months: 2 })).format();
```
results in `2020-06-06T09:00:00+02:00` as well.

\
More confusing to me is, that using the `add` method works without problems:
```ts
dayjs('2020-04-07T09:00:00').add(2, 'M').format();
```
results in `2020-06-07T09:00:00+02:00`.

Start by reproducing the two examples using dayjs.duration('P2M'), dayjs.duration({ months: 2 }), and add(2, 'M'), then trace the duration handling and add integration. Done requires an agreed month-relative interpretation and consistent results for these cases, covered by regression tests.

Written by the indexing model from the issue text.