Convert `_data/supporters.json` to a YAML file

[chastell]

The (strict, no-trailing-commas) JSON format makes merging concurrent PRs adding themselves to the end of it not possible without manual intervention.

Given that we use YAML in other places (translations’ frontmatter) and don’t use JSON for anything else I would be happy to migrate the file from JSON to YAML. This would make adding new supporters much easier.