[vscode] Implement Chat and Language Model initial APIs

[rschnekenbu]

VS Code 1.90 introduced the extension support for Chat (ChatParticipant) and Language Models

• see https://code.visualstudio.com/api/extension-guides/language-model
• see https://code.visualstudio.com/api/extension-guides/chat
• see https://github.com/microsoft/vscode/blob/5d6671dacb9d6a582b9354ea317211a8e2b2f918/src/vscode-dts/vscode.d.ts#L19190
• see https://github.com/microsoft/vscode/blob/5d6671dacb9d6a582b9354ea317211a8e2b2f918/src/vscode-dts/vscode.d.ts#L18580

[rschnekenbu]

This large API will be stubbed as an initial step, as it requires some root implementation and vscode extension support. A new issue has be opened to track real implementation of this API: #13777