Implement a resource bundler

[Julian]

We should offer a resource bundler which implements the "official JSON Schema bundling process". Doing so shouldn't be too difficult on top of what's already here.

Here's one from json-schema-ref-parser, and another from @jderosiers.

It's probably a judgement call whether to do this here in this repository or in a separate one which is JSON Schema specific (even though here we have referencing.jsonschema, a module for JSON Schema-specific functionality). But this kind of functionality relies on functionality which is less likely to be cross-spec applicable -- or maybe not and it can be (would probably involve adding some methods to Specification regardless which specify how to perform the bundling). Thinking required. If it does live here, it's probably something that can be a method on Registry, and may tangentially relate to #16.