This ensures we reply to "kernel_info_request" in a timely fashion, and
that we don't spawn a useless environment.  It also allows the user to
not repeat ";;guix run my-env" in subsequent cells since the default
environment is now recorded across cells.

* guix-jupyter-kernel.scm (%default-environment-property): New variable.
(proxy-state-default-environment, set-proxy-state-default-environment):
New procedures.
(create-environment): Call 'set-proxy-state-default-environment'.
(reply-execute-request): Use the environment returned by
'proxy-state-default-environment'.  Print an error as HTML when it's
false.
<top level>: Remove call to 'start-container' for the "default"
environment.