Use sub-directories for sub-regions

[Crissov]

Some ISO 3166-2 subregions for US and UK/GB are already included as US-##.{svg,png.html} and UK-###.*, respectively. BCP47 doesn’t care much about those, but UTR51.10+ does.

There are currently 5000ish subregion codes, many of which do not have a flag associated with them. Anyhow, this could easily get unwieldy in the directories if added in the future. Github Web “only” supports 1000 files per folder, if I remember correctly. I therefore suggest to move sub-region flags into sub-directories, i.e. effectively just replace the hyphen - in file names by a slash /.