Support --open with -r

We can open files in a running instance of code-server with `-r` (well technically `-r` is optional if code-server is already running) but it might be nice to also support `--open` to automatically launch the browser as well.

Only works if you are using code-server as a local editor but it would let you do things like implement "open with" without having to open the browser window ahead of time.

See https://github.com/coder/code-server/discussions/6224