Added base AI Panel component + AI navigation rail

What does this MR do and why?

• Adds base AI Panel component consisting of AI Chat Panel and Navigation rail
• Adds initial tests
• Preserves panel's state (collapsed or expanded) in local storage

Please note maximize button will be added back in later once we have designs for it

Screenshots or screen recordings

Before	After
	ai_panel_demo

How to set up and validate locally

Please make sure you have paneled_view and global_topbar (for the full experience) FFs enabled

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/562710