Add bottom panel to extension manager

[christian-byrne]

Allows extensions to manage the bottom panel. For instance, if an extension wants to show the terminal:

  const panel = app.extensionManager.bottomPanel
  const isTerminalVisible =
    panel.bottomPanelVisible &&
    panel.activeBottomPanelTab.id === 'logs-terminal'
  if (!isTerminalVisible) panel.toggleBottomPanelTab('logs-terminal')

The bottom panel support was added in #1294. Adding custom bottom panel tabs is possible through regsterExtension, but managing the panel is not easy for extensions.

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