feat: get author on series

lib/string.ts

@@ -100,3 +100,7 @@ const resourcePrefixes = Object.values(resources).map((v) => v.prefix).filter(
 );
 export const isLocalImage = (src: string) =>
   resourcePrefixes.some((p) => src.startsWith(p));
+
+export const isString = (input: string | undefined): input is string => {
+  return typeof input === "string";
+};