feat(design): support nested submenus in menu

[xelaint]

PR Checklist

• Commit message follows our contributing guidelines
• Tests added/updated (for bug fixes/features)
• Documentation added/updated (for bug fixes/features)

PR Type

• Bug fix
• Feature
• Style update
• Refactor
• Test
• Build
• CI
• Docs
• Performance
• Other (please describe)

Current behavior

Fixes: #4559

New behavior

• Add daffMenuActivator to a <daff-menu-item> to open a submenu from that item. Submenus open on hover by default. trigger="click" opens them on click instead.
• New trigger property to set opening submenus either on click or hover.
• Add keyboard interactions for submenu:
  • Right arrow opens submenu and moves focus into it
  • Left arrow returns to the parent item
  • Escape closes one level at a time
• A parent item shows a chevron and aria-haspopup="menu"
• Only one submenu of a menu is open at a time, and all open menus close when focus leaves them, on Tab, or on backdrop click

Breaking change?

• Yes
• No

Additional context