Add Luxembourgish (`lb`)
Add Luxembourgish (`lb`)

Part of BCP47: http://schneegans.de/lv/?tags=lb&format=text

```
Input
lb
Canonical
lb
Input is canonical
True
Well-formed
True
Valid
True
Registry date
2015-12-16
Primary language subtag
lb – Luxembourgish; Letzeburgesch
```
mozilla/language-mapping-list