Investigate to support 0-folder workspaces

[bpasero]

We should see if we could support workspaces with 0 folders.

/cc @sandy081

[bpasero]

Our code is quite fit for this, I only found the following that still need multi root adoption (via @dbaeumer work) that currently hardcode workspace.folders[0] usage:

• processRunnerDetector.ts
• processTaskSystem.ts
• task.contribution.ts

Once that is in we should remove the following checks for folders having entries:

• workspaceMainService#113
• Configuration#393

And then we can enable the "Remove Folder" action when only one root is there:

• fileActions.contribution.ts#95

And we need to update the JSON schema.

[sandy081]

I think we should also update the schema of workspace configuration file to allow no entries for folders

[dbaeumer]

The places where I use folders[0] is when I am sure that I am in a folder workspace.

[bpasero]

Verification: remove all folders from a workspace configuration file and verify you see no undesired behaviour. The explorer should show up as empty.