feat(REST API): Get cached time of Response

rest/getCachedAt.ts

@@ -0,0 +1,12 @@
+/** ServiceWorkerにcacheされた日時を得る
+ *
+ * cacheされたResponseでなければ`undefined`を返す
+ *
+ * @param res Response to check the chached date
+ * @return cached date (as UNIX timestamp) or `undefined`
+ */
+export const getCachedAt = (res: Response): number | undefined => {
+  const cachedAt = res.headers.get("x-serviceworker-cached");
+  if (!cachedAt) return;
+  return parseInt(cachedAt);
+};

rest/mod.ts

@@ -16,3 +16,4 @@ export * from "./error.ts";
 export * from "./getCodeBlocks.ts";
 export * from "./getCodeBlock.ts";
 export * from "./uploadToGCS.ts";
+export * from "./getCachedAt.ts";