Set session socket into environment variable (#6282)

* 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