feat: allow webviews in secondary side bar for extensions

[CamiKubaSilva]

Add support for contributing views to the secondarybar location

Summary

This PR enables extensions to contribute view containers and views to the Secondary Side Bar (secondarybar) using the existing viewsContainers and views contribution points.

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What this change does

• Adds "secondarybar" as a valid location for viewsContainers.
• Updates the extension point schema and handler logic to register view containers and views under ViewContainerLocation.AuxiliaryBar.
• Allows contributed views to be displayed in the Auxiliary Bar (aka Secondary Side Bar).

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Before

Only "activitybar" and "panel" were accepted locations for contributed view containers:

"viewsContainers": {
  "activitybar": [...],
  "panel": [...]
}

Extensions could not target the Secondary Side Bar for contributed views.

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After

Developers can now contribute view containers to the secondarybar:

"viewsContainers": {
  "activitybar": [...],
  "panel": [...],
  "secondarybar": [
    {
      "id": "my-view",
      "title": "My View",
      "icon": "media/icon.svg"
    }
  ]
}

Views inside this container will be rendered in the Auxiliary Bar.

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Motivation

Allowing extensions to render in the Secondary Side Bar makes it possible to separate content logically—for example, Explorer in the primary sidebar and a custom Assistant in the secondarybar—without UI conflicts.

This increases flexibility and helps support more complex UI workflows in advanced extension scenarios.