fix: [a11y] set aria-modal true when modal

[aversini]

Minor change to set “aria-modal” to “true” to the dialog container (the div that has the role of “dialog”), in the case of a modal. This is per WAI-ARIA practices for Dialog (Modal): https://www.w3.org/TR/wai-aria-practices-1.1/#dialog_roles_states_props

I didn’t add any stories to Storybook as it’s seems that adding a unit test for it might be enough… let me know :)