Fix event type

[terurou]

This PR fixes a compile error in the following code:

function activate(context:ExtensionContext) {
    final panel = Vscode.window.createWebviewPanel(
        "foo.bar",
        "Foo Bar",
        Vscode.ViewColumn.One,
        {}
    );
    panel.onDidDispose(
        _ -> {},
        null,
        context.subscriptions  // <- error
    );
}