chore: improve clsx maintenance path

[quyentonndbs]

Summary:

• Add targeted unit tests in test/index.js covering edge cases for the default clsx export: deeply nested mixed arrays containing objects with falsy values (0, '', null, undefined, false, NaN), arrays containing other arrays of objects, and symbol/bigint inputs (which should be ignored or coerced consistently). Each test follows the existing assert.is/assert.equal pattern, no behavior change required — only assertions of current observable behavior to lock it in.
• Keep the change narrow so it is straightforward to review.

Notes:

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