add missing src/properties-transformer.mjs from template

src/properties-transformer.mjs

@@ -0,0 +1,19 @@
+/**
+ * Creates a new transformer.
+ * On match the entry will be assigned new properties as given by propertyDefinitions.
+ * @param {Object} propertyDefinitions
+ * @param {Matcher} matcher
+ * @param {string} name
+ * @return {Transformer}
+ */
+export function createPropertiesTransformer(
+  match,
+  propertyDefinitions,
+  name = "property"
+) {
+  return {
+    name,
+    match,
+    transform: async entry => Object.create(entry, propertyDefinitions)
+  };
+}