LibPDF: Implement loading compressed objects from object streams

Now, whenever the xref table points to a compressed object,
parse_object_with_index will look it up in the corresponding object
stream as if it were a regular object.

With this, our parser gains the bare minimum support for xref streams.

Userland/Libraries/LibPDF/DocumentParser.h

@@ -85,6 +85,7 @@ private:
     PDFErrorOr<NonnullRefPtr<XRefTable>> parse_xref_stream();
     PDFErrorOr<NonnullRefPtr<XRefTable>> parse_xref_table();
     PDFErrorOr<NonnullRefPtr<DictObject>> parse_file_trailer();
+    PDFErrorOr<Value> parse_compressed_object_with_index(u32 index);
 
     bool navigate_to_before_eof_marker();
     bool navigate_to_after_startxref();