GEMS18 — Move normative schemas from the spec to separate machine-readable files

For the GEMS 1.7 RTF, the AB asked if we can move the normative schemas from the specification document to separate machine-readable files for the next GEMS RTF, and this ticket serves to capture that request.

This applies to the normative schemas in the XML and JSON PSMs; there are no applicable schemas in the ASCII PSM to be moved. Any non-normative snippets would remain in the specification as references or examples.