What problem does this feature solve?

The [folder] template string is supported in css-loader localIdentName option, but not supported in rspack native CSS.
Adding support for it would enhance usability.

config: rspack.Configuration = {
  module: {
    generator: {
      "css/auto": {
        localIdentName: "[folder]__[local]",
      },
    },
  },
};
// ex: `.root` in `a/b/c/TestRender/index.css` :
//  expected: `TestRender__root`
//  actual(current): `[folder]__root`

What does the proposed API of configuration look like?

webpack documentation: css-loader | webpack
crates/rspack_plugin_css/src/utils.rs might be involved.
https://github.com/web-infra-dev/rspack/blob/main/crates/rspack_plugin_css/src/utils.rs

original webpack patch:
feat: added [folder] placeholder
webpack-contrib/css-loader@a0dee4f
This commit includes the javascript implementation and tests.