add feature versions schema and tests

tests/helpers/schemas/feature-versions-schema.js

@@ -0,0 +1,13 @@
+const { schema } = require('../../../lib/frontmatter')
+
+// Copy the properties from the frontmatter schema.
+const featureVersions = Object.assign({}, schema.properties.versions)
+
+// Remove the feature versions properties.
+// We don't want to allow features within features! We just want pure versioning.
+delete featureVersions.properties.feature
+
+// Call it invalid if any properties other than version properties are found.
+featureVersions.additionalProperties = false
+
+module.exports = featureVersions