Extension host (#20)

* Implement net.Server

* Move Socket class into Client

This way we don't need to expose anything.

* Remove some unused imports

* Pass environment variables to bootstrap fork

* Add debug log for when socket disconnects from server

* Use VSCODE_ALLOW_IO for shared process only

* Extension host can send messages now

* Support callback for logging

This lets us do potentially expensive operations which will only be
performed if the log level is sufficiently low.

* Stop extension host from committing suicide

* Blank line

* Add static serve (#21)

* Add extension URLs

* how did i remove this

* Fix writing an empty string

* Implement dialogs on window service