Fetch IANA language subtag registry and format it

Summary:
Ref T1131

Test Plan:
Generate a database in the Darkbot format with:
./language-subtag-registry-datasource -l --format "lang+%%id%% %%fulldescription%%"

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: fauve

Maniphest Tasks: T1891, T1131

Differential Revision: https://devcentral.nasqueron.org/D3156