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](https://gitlab.com/gitlab-org/gitlab-web-ide-vscode-fork/-/blob/5f4e8d78cc4bef00d75ec56fac22eee3d35bac93/scripts/gl/editor-injector/gl_start_server.sh#L23)) - 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 https://gitlab.com/gitlab-org/gitlab-ui/-/merge_requests/3442+
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