support background reconnecting

[ericpaulsen]

an end-user from a large customer gave this feedback:

One thing that's significantly worse about vscode-coder than the old vscode-plain-ssh-to-vm is that it doesn't seem to be capable of background-reconnecting. It needs me to reload the whole window, which, in the event of a brief network outage, can destroy my focus much more thoroughly than the old way did.

[code-asher]

I ran into this after investigating a different issue. I think this is behavior from Microsoft's VS Code Remote SSH extension and so there may be nothing we can do about it.