fix: ignore functions without mutating input objects

[Shubham-Padkonde]

Using ignoreFunction: true currently deletes function-valued properties from the input object. Besides modifying caller-owned data, this throws for frozen objects and turns functions in dense arrays into holes rather than the null values produced by JSON.stringify.

Let JSON.stringify handle function values returned by the replacer instead of deleting properties. This preserves the documented JSON-like behavior of ignoreFunction while retaining the original objects and arrays.

Three regression tests fail before the fix: nested-object mutation, frozen objects, and functions in arrays. All 103 tests pass afterward (node --test test/unit/*.js), and git diff --check passes.