fix(types): restore `ExistsFunction` shape to keep arrow-function wrappers assignable #2425

The type-guard overload added in v25.10.4 broke assignment of plain
arrow functions to `ExistsFunction`-typed variables (TS cannot infer
type predicates through multi-overload assignment). The predicate is
now declared inline on `i18n.exists` instead, so direct
`i18next.exists(k)` calls still narrow `k` to `SelectorKey` (#2364).
Custom wrappers wanting the narrowing can use `typeof i18next.exists`.