Implement simple versions & simple version ranges #301

As per #43, we are going to restrict versions and version ranges in the registry. We can use existing tools like the node-semver package's parseRange function to produce simple ranges from commonly-used operators like ^ and ~, but this isn't sufficient to cover the restrictions we'd like to have in place.

Therefore, as part of adopting simple versions and version ranges we should also implement parsing and validation functions that enforce this restriction. This issue tracks implementing those checks and including tests to cover them.