Allow working with an empty workspace

[tgodzik]

Is your feature request related to a problem? Please describe.

When editors are not opened in a particular workspace they will not, in most cases, send Metals a workspace directory, which will cause Metals server to not work at all even in a case of a standalone file or worksheet.

Describe the solution you'd like

We should allow some providers to work with no workspace folder whenever it makes sense.

Describe alternatives you've considered

We could send parent directory in case of standalone files, but that is not ideal and will not work in case of scalameta/metals-vscode#357

Additional contex

scalameta/metals-vscode#357
scalameta/metals#3079 (reply in thread)

Search terms

empty workspace standalone