Missing `Intl` global documentation for checkout extensions

[lemonmade]

The global API documentation for checkout extensions does not include the Intl globals that we support for the purposes of our localization feature. This includes both the developer docs, and the content we have in this repo.

cc/ @kumar303

[kumar303]

It's mentioned here briefly but, yeah, good catch. We should add it to the documented globals.

[sjanczy2]

Marking this one as potential to close due to age unless we still want to do this eventually

[kumar303]

Personally, I think the existing docs on Intl is good enough 🤷