Add Luxembourgish (lb) localization support to flutter_localizations

### Use case

Flutter currently does not support Luxembourgish (lb) as a built-in localization option. This makes it harder to offer native-language UI experiences to users in Luxembourg. I'm not aware of any existing package that provides full translations for Material, Cupertino and Widget components. Manual localization may be possible, but is inconsistent and doesn't scale well.

### Proposal

As a native speaker with experience in translating English interface messages to Luxembourgish, I've translated the Material, Cupertino and Widget .arb files to Luxembourgish and attached them as .txt files to this issue.

[cupertino_lb.txt](https://github.com/user-attachments/files/21867094/cupertino_lb.txt)
[material_lb.txt](https://github.com/user-attachments/files/21867095/material_lb.txt)
[widgets_lb.txt](https://github.com/user-attachments/files/21867093/widgets_lb.txt)