Update code-server, add gitlab-workflow and code CLI
- Add code-server extension gitlab-workflow with default URL https://gitlab.cvh-server.de (fixes #1 (closed))
- Update code-server to 4.12.0 (fixes #2 (closed))
- Add Visual Studio Code CLI mainly to enable making tunnels (fixes #3 (closed))
- Map "Page ↓" and "Page ↑" in bash to search-history-forward and -backward, respectively (fixes #4 (closed)).