Add smoke tests for `shiny.playwright.NavPanel`

[schloerke]

From https://discord.com/channels/1109483223987277844/1271090164357402626/1271090164357402626

[schloerke]

Other Nav panel requests:

• Rename _NavPanelBase class to _NavSetBase
• Document _NavPanelBase's public nav_panel() method.
• Additional request to change data_value init parameter to panel_value to match the ui.nav_panel(*, value=) parameter.
  • Since our parameter is still a positional arg, the rename should be fine.
• Lower NavPanel's position in the quartodoc config so that it's not as important.
  • Add comment about using the navsets to retrieve the NavsetPanel, rather than creating one directly.

[schloerke]

Fixed in #1602