HexEditor: Create a function to initialize a HexDocumentFile

This function is made from what composed `set_file()` (which now calls
the new function). It allows to create a `HexDocumentFile` without
calling the hackish `set_file(move(m_file))`.

Userland/Applications/HexEditor/HexDocument.h

@@ -76,6 +76,8 @@ public:
 
 private:
     explicit HexDocumentFile(NonnullOwnPtr<Core::Stream::File> file);
+    ErrorOr<void> initialize_internal_state();
+
     void ensure_position_in_buffer(size_t position);
 
     NonnullOwnPtr<Core::Stream::File> m_file;