Implement a `is_empty_schema()` function in `src/core/jsonschema`

[jviotti]

Similar to sourcemeta::core::is_schema, but to check schemas that are strictly semantically equivalent to the empty schema, such as:

• {}
• true

Of course, schemas only containing identifier keywords (like $schema) or $comment can be semantically equivalent to the empty schema too, but we want to keep it simple, at least for now.