* Make scripts/migration/renamings.js a valid JSON5 file.
* Add a schema for it in tests/migration/renamings-schema.json.
* Add a script to test one against the other, as
tests/migration/validate-renamings. It is a node.js script
that will exit 0 if the renamings file is valid, or 1 if it
is not (and print a not-very-helpful error message from which
is possible, with some effort, to deduce what the error is.)
