add: test for >2 char iana tags, tag list update script, updated tags

- test_language_warnings.py
  - add case for a valid >2 char language code
  - add case for invalid 3 char code - example of behaviour where if
    someone refers to ISO 639 and gets a valid tag but that's not
    compliant with the IANA RFC which uses the shortest available tag.
- subtags_updater.py
  - add script to simplify/formalise how the tag lists are prepared.
    I don't know how/where the original list was obtained but this
    produces a purely additive changeset consistent with a published
    changelist https://www.iana.org/assignments/lang-subtags-templates/lang-subtags-templates.xhtml
    so it should be about right.
  - add IANA language codes added to registry since list was originally
    committed 6+ years ago (2018-04-01)