chore: Update source of langtags.json?

[darcywong00]

The devtool parse-langtags consumes the WS-tech file "langtags.json".

fwiw, @tim-eves said it's probably better to use this API to get the released version:
https://ldml.api.sil.org/langtags.json

instead of pulling the master branch directly from the GitHub source.

I haven't compared the differences yet... (release is roughly every 6 months)

[darcywong00]

@mhosken - Is there an API to also get the langtags_schema.json?