Navigation Component: Improve the Back Navigation

[Copons]

• Explore if it's possible to automatize the back button label for nested menus.
  Right now consumers have to provide the label manually (or it defaults to "Back").

gutenberg/packages/components/src/navigation/menu.js

Lines 41 to 50 in 668a710

	{ parentMenu && (
	<MenuBackButtonUI
	className="components-navigation__back-button"
	isTertiary
	onClick={ () => setActiveMenu( parentMenu, 'right' ) }
	>
	<Icon icon={ chevronLeft } />
	{ backButtonLabel || __( 'Back' ) }
	</MenuBackButtonUI>
	) }

By traversing the Navigation children we should be able to infer the parent menu's title and automatically use it as back button label for its children menus.

Depends on #25246 exploration.

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• Add an onBackButtonClick handler.