ButtonMenu should expose "open" event

[petemill]

If a <leo-button-menu> only shows on some css condition (e.g. hover of a parent), when it opens it's easy for the user to mouse-out and hide the menu. If we exposed whether the menu is open or not (perhaps via an event), then the consumer could keep it visible whilst open.

cc @nullhook @fallaciousreasoning

[fallaciousreasoning]

Seems sensible to me - might also be sensible to expose some of the floating-ui config for changing the menu placement & behavior too