chore: add explicit feature for example

[h-a-n-a]

Summary

Temporarily fix the issue with examples by adding an explicit warning for the user end when trying to invoke an example from command-line

error: target `serve` in package `rspack_node` requires the features: `test`
Consider enabling them by passing, e.g., `--features="test"`

Rust analyzer understands required-features and it will append features to the running command, so basically it is ok for our daily use basis.

For the long-term speaking, it's not a good idea to place configuration in node binding which arouse a mix-up with lib
and cdylib. We will resolve this in the future.

Further reading

rust-lang/cargo#4663
rust-lang/rfcs#2887

[h-a-n-a]

It's not a feature, so I merged this with one approval