feat: control badge positioning on icon within ion-button

# Feature Request

**Ionic version:**
[x] **4.x**

**Describe the Feature Request**
`ion-button` with slottable `ion-badge` to indicate notifications.

**Describe Preferred Solution**
`ion-tab-button` already has this. From the [doc](https://ionicframework.com/docs/api/tabs):
```

Schedule
6

```

**Describe Alternatives**
Reference to how this can be done currently:
https://forum.ionicframework.com/t/how-to-add-a-badge-to-a-icon-inside-a-ion-button/70868/3

Start by comparing the ion-button component with the existing ion-tab-button badge behavior and the linked tabs documentation. Confirm how a slotted ion-badge should be positioned on an icon within ion-button, then verify that the resulting layout matches the requested notification-badge example.

索引モデルが issue の本文から書いたものです。