What problem does this feature solve?

Migrating a project that currently use an extends property requires a rewrite of the configuration using webpack-merge (as documented here)

Implementing the extends property will make migration transparent
It can also be an opportunity to rewrite a performance oriented builtin webpack-merge in rust

What does the proposed API of configuration look like?

Webpack documentation: https://webpack.js.org/configuration/extending-configurations/

module.exports = {
  extends: path.resolve(__dirname, './base.rspack.config.js'),
  entry: './src/index.js',
  output: {
    path: path.resolve(__dirname, 'dist'),
    filename: 'bundle.js',
  },
};