Tabster doesn't set FOCUS_VISIBLE_ATTR to element when navigating from content-editable area

[weishanmicrosoft]

Hi,

We needs to shift focus from content-editable area to Popover and let Tabster handle the setting of FOCUS_VISIBLE_ATTR. Please see method "registerElementIfNavigating()" from https://github.com/microsoft/fluentui/blob/master/packages/react-components/react-tabster/src/focus/focusVisiblePolyfill.ts

Current Tabster only supports to set FOCUS_VISIBLE_ATTR when pressing TAB or currentActiveElement is not input/textarea/content-editable element. Please see "_shouldTriggerKeyboardNavigation()" from https://github.com/microsoft/keyborg/blob/main/src/Keyborg.ts

Thank you.