fix: avoid trailing comma in jsonStringify when replacer skips last property

[Suyog241005]

Description

This PR fixes a bug in jsonStringify where omitting the last property of an object via a custom replacer function resulted in a trailing comma (producing invalid JSON, e.g. {"aaa":"foo",}).

Instead of using the raw entries index (entries[index + 1]) to append commas (which doesn't account for properties that get filtered out by the replacer), stringifyObject now dynamically prepends commas before writing any subsequent non-undefined property.

Key Changes:

• lib/common.js: Refactored stringifyObject to dynamically prefix commas and converted the loop to a modern for-of loop to satisfy the project's @typescript-eslint/prefer-for-of linting rule.
• lib/stringify.spec.ts: Added a dedicated unit test case validating that omitting the last property produces correct, valid JSON without a trailing comma.

Related Issues

Closes #119

[Suyog241005]

Hi @jdesrosiers,

I've opened this PR with the fix.

It dynamically prefixes commas in stringifyObject to avoid trailing commas when properties are omitted. I also refactored it to a modern for-of loop to comply with the project's linter rules.