feat: support uniqBy function

[XieChengYi]

Summary

• add uniqBy(array, predicate) as a predicate builtin
• compile uniqBy through the VM loop/accumulator path, preserving first item per unique key
• add safe hash fast path for common keys with runtime.Equal fallback for complex keys
• document uniqBy and cover it with builtin and public API tests

Tests

• go test ./...

[XieChengYi]

Hi maintainers, thanks for the great project.

This PR adds uniqBy(array, predicate) as a natural extension of the existing uniq builtin, following the same predicate builtin pipeline as groupBy and sortBy.

I would appreciate any feedback on the API name, docs wording, or implementation approach. Happy to adjust it to better fit the project.
@antonmedv