question(common): Discuss whether to allow UN M49 region codes in language tagging

Martin Hosken feels we should not allow these. One examples is `yue-latn-001` where `001` is a global code and adds no value. However, other keyboards may be for Latin America and perhaps add value.
The list of codes is here:
https://en.wikipedia.org/wiki/UN_M49

The list of language tags using these codes is extensive and in a comment below.

One other issue is that the SIL langtags.json does not include these 3 letter numeric codes. If we feel they are important, should they be added to `langtags.json`.

That was a search for `Language ID=".+?\d+.+?"` against *.kps which should hopefully capture most of it.

Remember that lang tags also need to match against lexical models