Support multi root workspaces

Hello,

I've added multi root workspaces support to this VS Code extension.

Main changes:

• Commands and status bar:
• If workspace has only one root folder, then nothing should change for the user.
• If the workspace has many root folders, but none of them is a GitLab project folder, then nothing should change for the user.
• If the workspace has many root folders and some of them are GitLab projects, then it will execute GitLab commands based on the open file:
  • If the file is part of a GitLab project, then all commands will be related to this project
  • If the file is not part of a GitLab project, then a GitLab project selector will be shown to the user
    • If only one project is a GitLab project, then no project selector will be shown and it will default to the only GitLab project
• Gitlab sidebar:
  • If workspace has only one root folder, then nothing should change for the user.
  • If the workspace has many root folders, but none of them is a GitLab project folder, then nothing should change for the user.
  • If the workspace has many root folders and one of them is a GitLab projects, then it will show GitLab sidebar like that GitLab project was a single root folder
  • If the workspace has many root folders and some of them is a GitLab projects, then it will show GitLab sidebar with a tree of projects, and for each projects it will show the project info (See screenshot).
  • If the workspace has many root folders the "For current branch" panel will show info based on the GitLab project of the current open file.

This MR closes #90 (closed)