Move help and plot viewers to the VS Code Secondary Side Bar

Is your feature request related to a problem? Please describe.

The help and plot viewers open in the editor region, which isn't the most optimal use of screen space. In the default layout, the VS Code Panel is at the bottom and spans to the right edge of the window. This often leaves the bottom-right region unused (unless very long lines are sent to the terminal, which is not good style).

Describe the solution you'd like

Move help and plot viewers to the VS Code Secondary Side Bar. This would make better use of screen space, as the Secondary Side Bar spans to the bottom edge of the window.

Describe alternatives you've considered

Different layout combinations, e.g. Panel at the right, help and plot viewers at the bottom. None of these options seemed as effective as using the Secondary Side Bar.

{
    "workbench.panel.defaultLocation": "right",
    "workbench.editor.openSideBySideDirection": "down"
}

Additional context

Positron shows help and plots in the Secondary Side Bar. See e.g. here: https://positron.posit.co/features.html

Start by tracing the help and plot viewer entry points in the extension and how they are registered with VS Code. Compare their current editor-region placement with the Secondary Side Bar behavior described in the issue and the linked Positron reference. Done means both viewers open in the Secondary Side Bar without losing their existing functionality.