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

[wxiaoguang]

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

[keithamus]

Great catch! PRs welcome for sub components to call super.

[wxiaoguang]

I haven't gotten time for it, feel free to fix it by your plan.