SlButton does not re-render the disabled attribute correctly

When dynamically setting a button's `disabled` property, there's a visual bug where the inner button still appear disabled.

Example:
```javascript
{ this._disableButton = !this._disableButton; }}>Toggle disabled

Button test disabled=${this._disableButton}
```

Suggested fix:
```javascript
class SlButton extends LitElement {

...

get _attrs() {
return this.getAttributeNames().reduce((acc, name) => {
if ((name === 'class' || name === 'label' || name === 'disabled')) return acc;
acc[name] = this.getAttribute(name);
return acc;
}, {});
}

render() {
return html`




`;
}
}
```

Locate the SlButton implementation and start by reproducing the dynamic disabled-property example from the issue. Compare the host attribute with the inner button state; done means toggling the property updates the inner button's disabled state correctly.

Written by the indexing model from the issue text.