Add JSON converter

[kg583]

A straightforward JSON converter function, which doubly serves as a partial format validator. This would be called by CI to deposit the converted sheet in the built branch. The file name anticipates more formats, but there are no others I'd advocate for at this very moment.

The JSON format makes unique tags into keys and collects everything else into lists. Each token is a list of version dicts, and each translation lives in the version's langs dict, indexed by language code. The list of variants is not present if it is empty (okay with changing this).

[kg583]

The past three commits add a rather beefy validator function for the primary sheet, which does more than a static XML schema could with regards to checking tag contents, name uniqueness, etc. It's a little tricky to test given that the sheets currently pass, but some dummy edits have proven its at least not doing nothing. Detailed errors on which token fails validation and how are provided at every opportunity.