Buttons do not support aria-expanded

[steverep]

Describe the bug The APIs for mwc-button and mwc-icon-button have no way to set the aria-expanded attribute on the <button> element. This is needed, for example, when the button is used to open a popup menu. It should be set to true or false based on user input, and omitted when null or the empty string.

To Reproduce n/a

Expected behavior Button components should support all potential ARIA attributes as defined in the W3C spec and transfer those attributes and states to the internal <button> element.

Screenshots n/a

Browser Version (please complete the following information): n/a

Additional context ARIA 1.2 Authoring Practice for menu buttons