Main menu accessibility

[fschroiff]

Describe the bug
Slack discussion

The main menu implements a creative solution for keyboard accessibility. It is not very discoverable though and inaccessible for screen reader users.

To Reproduce
Steps to reproduce the behavior:

1. Go to fronteers.nl on desktop
2. Tab to the main menu
3. Using left and right arrow keys you can jump between main menu items
4. Using up and down arrow keys you can go into the sub-menus

Expected behavior
Good examples for accessible keyboard nav with drop downs:
https://a11y-collective.github.io/demos/en/advanced-accessible-components/components/navigation/navigation.html
https://www.eur.nl/

• everything accessible by tab
• the "open dropdown" action is triggered by activating a button
• you can tab through the open submenu
• tabbing out of the submenu closes it (tab & shift-tab)
• esc also closes the open submenu
• the submenu trigger has an aria-label of "Submenu of [xyz]" and an aria-expanded attribute