chore: improve clsx maintenance path

[quyentonndbs]

Summary:

• Add focused unit tests in test/index.js covering edge cases of clsx(): deeply nested mixed arrays/objects, falsy values inside object keys, numeric 0 handling, and ensure the existing src/index.js behavior is preserved (no behavior change, tests-only).
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.