Opening a new file spawns a new HLS instance

In VSCode, when opening a new file via `ctrl+n`, vscode-haskell spawns a new HLS instance for the file that hasn't been persisted to the file system yet and is only available in the VFS.

The only way to fix that is to save the file to disk and then restart the extension.

Reproduce the issue in VS Code by opening a new file with ctrl+n, then trace how the extension handles unsaved VFS documents and HLS instance creation. Done means opening an unsaved file reuses the appropriate HLS instance without requiring the file to be saved or the extension restarted.

Written by the indexing model from the issue text.