Issue #3615450: Replace the custom sklon() pluralization with core APIs

Adds MyrestHelperInterface::formatCount(), a wrapper over the core plural formatter, so the plural rules of the current language choose the variant and both variants can be translated through the interface translation UI.

sklon() is deprecated for removal in myrest:2.0.0 but keeps its behaviour: its suffix array holds three Slavic forms against the formatter's two, so a delegating shim would return a different form for counts like 5 on a site with no translation loaded. It had no call sites in the module.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com

Closes #3615450