Add TypeScript definition

index.d.ts

@@ -1,4 +1,22 @@
-export default function getPort(options?:{
-	port?: number | number[],
+/**
+ * Options to configure how to find an available port.
+ */
+export type Options = Readonly<{
+	/**
+	 * A preferred port or an array of preferred ports to use.
+	 */
+	port?: number | ReadonlyArray<number>,
+
+	/**
+	 * The host on which port resolution should be performed. Can be either an IPv4 or IPv6 address.
+	 */
 	host?: string;
-}): Promise<number>;
+}>;
+
+/**
+ * Returns a `Promise` for an available port number.
+ *
+ * @param options
+ * @returns Available port number.
+ */
+export default function getPort(options?:Options): Promise<number>;