Fix inconsistency between JSON schema and code for secondary sidebar view containers

[Copilot]

Extension developers were required to use "secondarySideBar" (uppercase B) in their package.json to register view containers in the secondary sidebar, but this caused JSON schema validation errors because the schema expected "secondarySidebar" (lowercase b).

The issue was caused by an inconsistency between:

• JSON schema definition: 'secondarySidebar' (lowercase b)
• Switch case logic: 'secondarySideBar' (uppercase B)
• API proposal name: 'contribSecondarySideBar' (uppercase B)

This fix makes everything consistent by using secondarySidebar (lowercase b) throughout:

1. Updated the switch case from 'secondarySideBar' to 'secondarySidebar' in viewsExtensionPoint.ts
2. Changed API proposal name from 'contribSecondarySideBar' to 'contribSecondarySidebar' in extensionsApiProposals.ts
3. Renamed contribSecondarySideBar.d.ts to contribSecondarySidebar.d.ts and updated the URL reference

Now extension developers can use the correct schema-compliant syntax without validation errors:

{
  "contributes": {
    "viewsContainers": {
      "secondarySidebar": [
        {
          "id": "myViewContainer",
          "title": "My View Container",
          "icon": "$(symbol-class)"
        }
      ]
    }
  },
  "enabledApiProposals": ["contribSecondarySidebar"]
}

Fixes #264529.

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