Hi all,

First, thanks for creating Rspack!

I'm trying to from Webpack to Rspack and encountered an issue with CopyRspackPlugin's compatibility with CopyWebpackPlugin.

The gist is that in CopyWebpackPlugin, pattern.transform accepts an object whereas CopyRspackPlugin does not (Note: this has been introduced in CopyWebpackPlugin@7)

// CopyWebpackPlugin
type transform =
  | {
      transformer: (input: string, absoluteFilename: string) => string | Buffer;
      cache?: boolean | TransformerCacheObject | undefined;
    }
  | ((input: string, absoluteFilename: string) => string | Buffer);

vs

// CopyRspackPlugin
type Transform = (
  input: Buffer,
  absoluteFilename: string,
) => string | Buffer | Promise<string> | Promise<Buffer>;

This lead to a cryptic error:

 #  rspack-node[77491]: napi_status napi_release_threadsafe_function(napi_threadsafe_function, napi_threadsafe_function_release_mode) at ../src/node_api.cc:1398
  #  Assertion failed: (func) != nullptr

I see two possible solutions:

1. Accept the API difference, and update the migration docs from Webpack to Rspack relating CopyWebpackPlugin
   a. possibly extended with some configuration validation which would throw a more informative error
2. Update CopyRspackPlugin to align the APIs

Minimal reproduction example: rstackjs/rstack-examples@main...patricksevat:rspack-examples:feat/copy-webpack-plugin-transform-obj-config