Function handle_editor_keydown 

fn handle_editor_keydown(
    ev: &KeyboardEvent,
    project: Option<RwSignal<Option<EditProject>>>,
    history: Option<StoredValue<Option<History>>>,
    selection: Option<RwSignal<Option<usize>>>,
    status: RwSignal<EditorStatus>,
)

Handle a keydown on the editor surface: transport (Space / arrows / I·O) and edit (S split, ⌘Z undo·redo, Delete ripple) shortcuts.