added generateCustomHeaders utility

src/generators/custom.ts

@@ -0,0 +1,14 @@
+import type { HeaderEntries } from "./index.js";
+
+export type CustomHeaders = { contributors: string; } & { [name: string]: string; };
+
+/**
+ * @summary generates custom headers not built-in to any of the managers
+ * @param custom list of custom headers
+ */
+export const generateCustomHeaders = (custom: string[]): HeaderEntries<CustomHeaders> => {
+    return custom.map((pair) => {
+        const [name, value = ""] = pair.split(/\s+/);
+        return [name, value];
+    });
+};