Support multiple roots in a workspace

[amisevsk]

Is your feature request related to a problem? Please describe.
We would like to add Scala support to Eclipse Che. Che uses the Theia editor, which supports the language server protocol used in VS Code.

In Che, we start workspaces in a remote container and clone in a user-specified repo. This means that the "open folder" in the editor is /projects, with the Scala project imported into a subfolder (e.g. /projects/my-scala-sample). When this occurs, metals fails to initialize, as there is no build.sbt in the root folder (/projects).

Describe the solution you'd like
Support the "workspace folders request", as defined in the language server protocol as of 3.6.0.

Describe alternatives you've considered
The current workaround is that the user must open the subfolder into which their project has been cloned; this is a significant departure from how other language servers handle this (e.g. Go and Java both initialize without this issue).

Additional context

• The PR for adding the plugin to Eclipse Che: Add Metals, the Scala language server eclipse-che/che-plugin-registry#356 (note it generally works out of the box, except for the workaround described above).
• A sample of how this parameter is handled in the JDT.ls case

Search terms:
workspaceFolders, initializeParams, language server protocol

[tgodzik]

Thanks for reporting! This is something we plan to do, but will require a bit of refactor, so not sure when exactly we will be able to fix this.

It is something that I keep in my mind, as it would be useful in a number of scenarios.

This could also be a workaround for: #5907