feat: show welcome view when there is no git repository

The workspace might not contain a repository for a variety of reasons:

• there is no open folder in VS Code (an empty workspace)
• the open folder doesn't contain a repository
• user hasn't got git binary installed

Currently, we show a bunch of errors (see "before" in the table) (also see this bug report: #363 (closed)). With this change, we'll show only a short welcome screen asking the user to check the "Source Control" tab to find out why we can't show any GitLab projects (repositories). We can do this because we are now getting information about git repositories from the native git extension.

before	after

Open folder is not git repository

The git extension is disabled

Related to #345 (closed)