react-dialog: send focus to the dialog root when no other focusables are present

[smhigley]

(discussed during the dialog a11y pass)

Two steps needed to implement the fix:

• the dialog surface (the element with role="dialog") should have tabindex="-1" to make it focusable, but not in the tab order
• When no other focusables are present, focus should go to the role=dialog element

This will fix the following bugs:

• Opening a dialog with no focusables gives no screen reader feedback
• A modal dialog with no focusables fully breaks the page for screen reader users
• Keyboard users cannot dismiss a dialog with no focusables, since escape is implemented within the dialog itself