chore: improve clsx maintenance path

[quyentonndbs]

Summary:

• Add edge-case unit tests in test/index.js covering currently-untested inputs to clsx(): deeply nested arrays mixed with falsy values (null, undefined, false, 0, ''), Symbol values (should be ignored / produce no class), and objects with inherited enumerable keys (verify own-property behavior). These exercise existing branches in src/index.js without changing behavior, improving regression coverage.
• Keep the change narrow so it is straightforward to review.

Notes:

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