Add support for cssFilename to NxAppRspackPlugin

[JPrisk]

When using NxAppRspackPlugin, add functionality to be able to specify custom cssFilename for css files in the same way that the filename can be customised for bundled js files.

References:

• https://rspack.rs/config/output#outputcssfilename
• https://github.com/nrwl/nx/blob/master/packages/rspack/src/plugins/utils/apply-web-config.ts#L307-L314
• https://github.com/nrwl/nx/blob/master/packages/rspack/src/plugins/utils/apply-base-config.ts#L143-L148

[JPrisk]

Opened PR here: #32506