add missing src/transform.mjs from template

src/transform.mjs

@@ -0,0 +1,36 @@
+export * from "./properties-transformer.mjs";
+export * from "./expression-transformer.mjs";
+export * from "./matcher.mjs";
+
+import { ContentEntry} from "content-entry";
+
+/**
+ * Apply transformers.
+ * @param {AsyncIterable<ContentEntry>} source
+ * @param {Transformer[]} transformers
+ * @param {boolean} onlyMatching filter out all none matching entries
+ */
+export async function* transform(source, transformers = [], onlyMatching) {
+  const usedTransformers = new Set();
+
+  for await (let entry of source) {
+    let didMatch = false;
+    for (const t of transformers) {
+      if (t.match(entry)) {
+        didMatch = true;
+        entry = await t.transform(entry);
+        usedTransformers.add(t);
+      }
+    }
+
+    if (didMatch || !onlyMatching) {
+      yield entry;
+    }
+  }
+
+  for (const t of transformers) {
+    if (!usedTransformers.has(t) && t.createEntryWhenMissing !== undefined) {
+      yield t.transform(await t.createEntryWhenMissing());
+    }
+  }
+}