Inconsistent Duration Units

Currently there seems to be some inconsistency and duplication of units when parsing month/week durations:

**Current Behavior**

```javascript
dayjs.duration('P1M').months(); // "1"
dayjs.duration('P1M').weeks(); // 4

dayjs.duration('P4W').weeks(); // 4
dayjs.duration('P4W').months(); // undefined

dayjs.duration('P1M4W').months(); // "1"
dayjs.duration('P1M4W').weeks(); // 8
```

Months are always converted into 4 weeks (additionally to the `month` unit also being set).

This behavior is not congruent with other units:

```javascript
dayjs.duration('P1Y').years(); // "1"
dayjs.duration('P1Y').months(); // undefined
dayjs.duration('P1Y').weeks(); // 52

dayjs.duration('P12M').years(); // undefined
dayjs.duration('P12M').months(); // "12"
dayjs.duration('P12M').weeks(); // 51
```

**Expected Behavior**

```javascript
dayjs.duration('P1M').months(); // 1
dayjs.duration('P1M').weeks(); // undefined

dayjs.duration('P4W').weeks(); // 4
dayjs.duration('P4W').months(); // undefined

dayjs.duration('P1M4W').months(); // 1
dayjs.duration('P1M4W').weeks(); // 4
```

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

Start by locating the duration parsing and unit-conversion entry points, then inspect the existing duration tests for month and week cases. The work is done when the reported examples match the expected behavior without changing unrelated duration units.

Written by the indexing model from the issue text.