rspack should export `hotLoader` of css extract plugin

[LingyuCoder]

Some HMR test cases of css extract plugin need this loader, but it is not exported from rspack

https://github.com/rspack-contrib/rspack-plugin-ci/blob/a3809a14346887d77a6e4fe18ba9571b925ae653/css-extract/HMR.test.js#L368-L391