New commands to cycle between open plugin panes

What

Plugin panes (side panels that plugins can open) gained navigation support:

• Two new commands, pane:next and pane:previous ("show next plugin pane" / "show previous plugin pane"), let you cycle between multiple open panes, alongside the existing pane:grow/pane:shrink/pane:close commands.
• A tab strip component shows the other open panes with a way to pick one directly.
• A new closeOnEscape pane option, a dock "tail" indicator element, and a toast-hold hook were also added to support this.

Why

This makes it practical to work with several plugin panes open at once, giving you a quick way to switch between them instead of only being able to close or resize one at a time.