MenuItem keyboard selection

[mimarz]

We currently don't have a clear example on how MenuItem is selected/triggered when pressing Enter key as it also triggers the onClick (due to event bubbling) and causes some unwanted behaviours.

Notes:

• Should we expect users to add key handlers on MenuItem like they do with onClick?
  • Maybe we just need to add a shim for event bubbling for keystrokes (like we have for MenuItem.onClick)
    • This might break anchor element focus on close?
• https://www.w3.org/TR/wai-aria-practices-1.1/examples/menu-button/menu-button-links.html