Add `open` command to `@vscode/dev-container-cli`

This issue is a follow-up of https://github.com/microsoft/vscode-remote-release/issues/5415#issuecomment-981373936.

Now the devcontainer cli is available to be installed from npm with npm install --global @vscode/dev-container-cli, but it's missing the open command.

My use case is to have it installed as part of my dotfiles, so that I get the devcontainer command installed every time I bootstrap a new computer with my dotfiles installation script (which also manages to install VS Code, by the way).

However, I use a similar dotfiles experience as a managed development environment for the my team, and there, I have a guide where I tell them to:

1. Install our dotfiles to bootstrap their dev env
2. Clone any of our company's repository
3. devcontainer open it

But it would be too cumbersome if I had to write instructions like:

1. First open VS Code
2. F1 -> Install Devcontainer CLI
3. Answers the questions on the screen

And then get back to the part where devcontainer open would actually work.