Returns the connection registered with
set_default_connection(), or one constructed from the seatabler.url,
seatabler.token_envvar and seatabler.workspace_id options. Errors with
guidance if no server URL is available.
Generic seatable_* functions default their con argument to this, so an
interactive user can set a default once and then omit con. Wrapper
packages instead pass an explicit connection so their end users never need
to configure anything.