Standalone CLI: add install extension --pre-release extension

[alexweininger]

We need the VS Code standalone CLI to be able to install pre-release extensions. Specifically, we are using the tunnel command, like so: code-insiders tunnel --name cloudshell... --install-extension Github.copilot-chat but we need to be able to add the --pre-release flag like you can do on the non-standalone CLI.

For example: code-insiders tunnel --name cloudshell... --install-extension Github.copilot-chat --pre-release

[sandy081]

This is supported already

code-insiders tunnel --name cloudshell... --install-extension Github.copilot-chat@prerelease

[alexweininger]

Thank you!