requests.rs : remplace usage of CSSlsrs' store by WEBlsp own store

See `fn get_language_id(` in `request.rs`, used to dispatch the request to the appropriate language handler based on the current document's language id.

For now, we use CSSlsrs' store, because we only support CSS. So I can just retrieve the document from this store from its URI. Soon, WEBlsp will support multiple languages, and have its own store. The document will be retrieved from this store instead, and the stores for each language will be created with `LANGlsrs::new_with_store()` instead of `LANGlsrs::new()`.