Built-in modifier that implements the N sigil modifier's
pluralization logic.
Looks up :count in the bindings and, if present, returns
{:ok, {msgid, msgid, count, remaining_bindings}} so the sigil macro
emits a dpngettext/6 call. If :count is absent, returns an error
that the sigil macro raises as a compile-time ArgumentError.
See the Pluralization guide for the full semantics and examples.