The `role` code is never executed in base `MarkdownButtonElement`

The `role` code is never executed in base `MarkdownButtonElement`

```
    connectedCallback() {
        if (!this.hasAttribute('role')) {
            this.setAttribute('role', 'button');
        }
    }
```

Because other classes like `MarkdownHeaderButtonElement extends MarkdownButtonElement` never call parent class's `connectedCallback`