* Avoid spawning code-server with --reuse-window and --new-window
These flags mean the user explicitly wants to open in an existing
instance so if the socket is down it should error rather than try to
spawn code-server normally.
* Set session socket into environment variable
While I was at it I added a CLI flag to override the default. I also
swapped the default to --user-data-dir.
The value is set on an environment variable so it can be used by the
extension host similar to VSCODE_IPC_HOOK_CLI.
* Add e2e test for opening files externally
