vscode-dotty: auto-configuration doesn't work when a file is opened outside a workspace

[smarter]

• Launch code
• File -> Close Folder
• File -> New File
• Save as foo.scala
• Get a notification "This looks like an unconfigured Scala project. Would you like to start the Dotty IDE?", answer yes.

Then nothing happens, the root cause can be shown by doing:

• Help -> Toggle Developer Tools
• Click on the "Console" tab in the panel that appeared
• See the error message lampepfl.dotty]ENOENT: no such file or directory, mkdir 'undefined/project'

We should either support running the extension outside of a workspace (is there an API to create our own workspace ?) or not try to run the extension when the workspace is undefined.

[Duhemm]

I have a possible fix in #5285, but I'm a bit worried by the reloading that gets triggered, because I haven't seen that documented anywhere.