This implements most of the CloseWatcher API from the html spec.
AbortSignal support is unimplemented.
Integration with dialogs and popovers is also unimplemented.
(cherry picked from commit b216046234560df531e1a32269e5dfa18f8f240c,
manually amended to replace a single `UIEvents::KeyCode::Key_Escape`
with `KeyCode::Key_Escape` in EventHandler.cpp since we don't have
the second commit of https://github.com/LadybirdBrowser/ladybird/pull/86)
