feat: (iso.ts) Add ISODuration and update doc blocks

lib/types/iso.ts

@@ -1,13 +1,16 @@
 import type { Nominal } from './nominal';
 
-/** A string formatted as [ISO 3166-1](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2).*/
+/** A string formatted as an [ISO 3166-1](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2) country code.*/
 export type ISOCountry = Nominal<'ISOCountry', string>;
 
-/** A string formatted as [ISO 639-1](https://en.wikipedia.org/wiki/ISO_639-1).*/
+/** A string formatted as an [ISO 639-1](https://en.wikipedia.org/wiki/ISO_639-1) language code.*/
 export type ISOLanguage = Nominal<'ISOLanguage', string>;
 
 /** A string formatted as `xx-YY`, where `xx` is an {@linkcode ISOLanguage} and `YY` is an {@linkcode ISOCountry}. */
 export type ISOLocale = Nominal<'ISOLocale', string>;
 
-/** A string formatted as [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601).*/
+/** A string formatted as an [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) date and time.*/
 export type ISOTimestamp = Nominal<'ISOTimestamp', string>;
+
+/** A string formatted as an [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601#Durations) length of time.*/
+export type ISODuration = Nominal<'ISODuration', string>;