Add support for `isv` as valid language tag

[carlosjeurissen]

Product

axe-core

Feature Description

Interslavic (with code isv), is currently regarded as an invalid language code. Even though it is a valid BCP47 language tag.

https://github.com/dequelabs/axe-core/blob/develop/lib/core/utils/valid-langs.js

[WilcoFiers]

@carlosjeurissen Thanks for raising this. Looks like axe-core's copy of the language tag registry is a couple years old. isv was added to the language tag registry in 2024.