Webpack config: provide raw asset loading rule #837

There are a few places in the frontend codebase that use the Webpack-specific raw-loader to load assets, for one reason or another. (For example, in frontend-app-discussions.) We should instead be using Asset Module Syntax, as it's more similar to what Vite and Rsbuild are doing, and it doesn't require an additional dependency. (I.e: it'll make our lives easier when porting.)

This was implemented locally in https://github.com/openedx/frontend-app-communications/pull/193/files, but we should provide the asset rule globally, in the default webpack configuration.