Ensure python.pythonPath is set for LSP configuration/workspace call

[jakebailey]

With python.pythonPath no longer always being set, a language server which queries for the user's configuration will no longer be able to see the currently active environment, as LSP only queries VS Code's view of the settings.json overlays.

To keep this call working, the old python.pythonPath setting can be added to the response to the workspace/configuration call, simulating the old behavior while keeping the value stored outside any settings.json.

Part of #11015.

[jakebailey]

This has been done for ages.