Hookup LanguageModelDataPart to chat ouput renderers

For https://github.com/microsoft/vscode/issues/257761

Let's hook up data parts so they can be rendered by chat output renderers as well. That should allow extension to generate content that is rendered directly instead of relying on tool calls:

```ts
new LanguageModelDataPart(encodedMermaidText, 'text/vnd.mermaid')
```
