Summary

This PR aims to improve DX by narrow the type of Rspack.Configuration to make it easier to use tools.rspack function.

These properties are set by Rsbuild by default so they are non-nullable. A possible edge case is that a plugin removes these fields, but this is unlikely to happen.

• With Rspack.Configuration, the plugins property is nullable:

tools: {
  rspack(config) {
    if (!config.plugins) {
      config.plugins = [];
    }
    config.plugins.push(new SomePlugin());
  }
}

• With NarrowedRspackConfig, the plugins property is non-nullable:

tools: {
  rspack(config) {
    config.plugins.push(new SomePlugin());
  }
}

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).