feat(iconbutton): Add `.mdc-icon-button--display-flex` class that centers icon via flexbox. When using the new theme API, the icon button should have this class.
feat(iconbutton): Add `.mdc-icon-button--display-flex` class that centers icon via flexbox. When using the new theme API, the icon button should have this class.

PiperOrigin-RevId: 396890519

material-components/material-components-web,material-components/material-components-web,material-components/material-components-web