Menu List component could use a11y improvements

Description

The menu list component could benefit from some a11y improvements.

Steps to reproduce

1. Visit https://protocol.mozilla.org/patterns/molecules/menu-list.html
2. Using the keyboard, tab so the menu list is focused.
3. Hit enter to open the menu.

Expected result

1. If i hit esc when open, it should close the menu.
2. If I click anywhere outside of the menu whilst open, the menu should close again.
3. If I tab through the menu items and past the last item in the list, then the menu should close when focus leaves the element.

Actual result

The menu remains open for all of the above items. The only way to close a menu currently is to click the title again.

Environment

Firefox Nightly (but happens in any browser)

Start with the menu list component shown on the linked Menu List page and reproduce the keyboard and focus behavior in the issue. Check how the menu responds to Escape, outside clicks, and focus moving past the last item. Done means each action closes the open menu consistently across browsers.