openforcefield/openff-forcefields: Version 2.0.0-rc.1 "Sage" Release Candidate

The contents of this major release candidate are adapted from https://github.com/openforcefield/openff-sage/releases/tag/2.0.0-rc.1. See that release's notes and assets for more details. Provided that early use of this force field shows no major deficiencies, this may be promoted to the full Sage release