Add devfile for gitlab-vscode-extension

To improve dogfooding, add a devfile for gitlab-vscode-extension.

• Add a script that will do the following
  • Check if you already have VS Code version specified already downloaded. If not, download it in a directory inside the repository.(will obviously be git-ignored)
  • Start a VS Code server by specifying the host, port and extension directory(the repository code itself) (example)
• Whenever someone wants to test something, they just run the script which will setup everything.
• Then the devfile would be simple just like we have in chore: Add devfile for workspaces (gitlab-ui!3442 - merged)