This PR adds a new module providing an implementation agnostic JSON serializer. The intention behind this PR is to allow platforms to swap between or use alternative implementations of a json serializer (see #839) whilst still allowing consumers of the platform to work regardless of the backing impl.

work that needs doing:

• decide whether we need to bother with legacy stuff (legacy nbt serializer too?)
• common tests for other serializers to use
• common jmh benchmarks for other serializers to use
• better error message for missing impl
• docs for this (also in javadocs)
• common builder for JSON serializers
• finish moving text-serializer-gson-legacy-impl out to a json version, remove the old one from the bom, and stop publishing it with 4.15.0 (anyone still using it can just depend on an old version)
• [ ] do we want a common flavour of the Provider services that normal serializers have, intended to be implemented by platforms? not worth doing in this first revision