Support --open with -r

[code-asher]

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 #6224