Use jQuery.i18n from MediaWiki core if available.

This allows other extensions to use this too, without needing a hack to detect
ULS's existence and work around it by not loading their copies of j/i18n code.

Preserves backwards-compatibility for archaïc versions of MediaWiki.

Bug: T95666
Change-Id: I25b33437ceddb60570322b9aa51844acc42a3e32