fix(ui5-form-item): update host element display

[ilhan007]

The isElementHidden method called internally by getFirstFocusableElement returns true on Safari for the ui5-form-item, considering the element as hidden, skipping the traverse of its children. This happens, because el.offsetHight and offsetWidth for the host custom element (ui5-form-item custom element in the specific issue) are 0 on Safari.

This probably happens for other components - Safari's layout engine doesn't always calculate dimensions correctly for the host element when content is in shadow DOM.

Solution - add display to the host element:

:host(:not([hidden])) {
	display: block;
	width: 100%;
}

Fixes: #12201