Add support for publishing to the PureScript registry

[thomashoneyman]

Pulp users can't use the PureScript registry until Pulp adds support for a solver and downloading packages from the registry. However, Pulp users can still publish to the registry using the registry's legacy mode.

Pulp should either update the publish command to push to both the new registry and to Pursuit, or add a new command that allows users to generate a .purs.json file and publish to the new registry. That way, current Pulp users can still publish their packages even if they aren't yet using the new registry.