Add `toggle` and `beforetoggle` events for the `popover` API.

[SylvainBoilard]

Hello,

This PR initially added support for toggle event handlers in details elements and was extended to support both toggle and beforetoggle events for the popover API.

See:

• https://html.spec.whatwg.org/multipage/popover.html ;
• https://html.spec.whatwg.org/multipage/indices.html#event-toggle ;
• https://html.spec.whatwg.org/multipage/interactive-elements.html#the-details-element ;
• https://developer.mozilla.org/en-US/docs/Web/API/Popover_API ;
• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details#events ;
• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details#attributes .

See also these related PRs:

• #341 in tyxml
• #1728 in js_of_ocaml
• #1734 in js_of_ocaml

Please let me know of any issues.

Cheers,