Finalize notebookKernelSource for third-party notebook extensions

We maintain the [marimo VS Code extension](https://github.com/marimo-team/marimo-lsp). Without `notebookKernelSource`, we must register every discovered Python environment as a controller, which makes the kernel picker noisy.

This API would let us expose “Python Environments” and “marimo sandbox” as sources and create controllers only when selected, matching the Jupyter experience. Would VS Code consider finalizing it? We’d be happy to provide an implementation sample and feedback.