Add support for objects with custom toString

[sjh9714]

Hi, this is a small follow-up for #52.

This adds support for objects/classes that provide a custom toString() method, matching the shape discussed in the issue. Normal object maps still use the existing key/value handling when the object has the default Object.prototype.toString.

I know this may not be the right tradeoff for clsx. In my local build, the output changed from 239 -> 277 B for dist/clsx.mjs, 245 -> 281 B for dist/clsx.js, and 305 -> 340 B for dist/clsx.min.js. If that size/perf tradeoff is not desirable, please feel free to close this PR.

I used Node 20.20.2 for tests and bench. On my local Node 22.22.0 setup, the existing uvu -r esm test command fails before reaching this change.

Testing

• npx -y node@20.20.2 node_modules/.bin/uvu -r esm test
• npm run build
• npx -y node@20.20.2 bench

Refs #52