Panel tab buttons should show currently selected state

[simonw]

It would be good if the buttons showed which one had been most recently selected here:

That's from this plugin:

document.addEventListener('datasette_init', function(ev) {
  ev.detail.registerPlugin('panel-plugin', {
    version: 0.1,
    makeAboveTablePanelConfigs: () => {
      return [
        {
          id: 'first-panel',
          label: 'First panel',
          render: node => {
            node.innerHTML = '<h2>My 1st panel</h2><p>This is a custom panel that I added using a JavaScript plugin</p>';
          }
        },
        {
          id: 'second-panel',
          label: 'Second panel',
          render: node => {
            node.innerHTML = '<h2>My 2nd panel</h2><p>This is a custom panel that I added using a JavaScript plugin</p>';
          }
        }
      ]
    }
  });
});

Originally posted by @simonw in #2250 (comment)