support for 4 letter locales to LangCode

[benllshua]

Goals

In order to expand globally, a lot of times developers and companies would like to use a 4 letter locale for their i18n.
the sitemap.js|ts currently support only 2 letter langauge code (en, ru, ar, he, sp...)
I want it to support also 4 letter locales (en-US, en-CA, he-IL, en-GB...)

sitemap.xml google crawlers support this alternates langauge code, next.js should allow those to.

Non-Goals

Background

I currently work on a big project that already configured his I18n and I'm working on their SEO, and I cant create a sitemap for them as next.js doesnt support it, instead of XML file it becomes weird broken txt when I try 4 letter locales (2 letter locales work fine)

Proposal

support 4 letter locales