feat(api): A feature flag system to handle version compatibility

[zml2008]

This system allows us to tailor output for different game/protocol versions.

This will probably want to become its own submodule, or likely standalone project in order to be usable in adventure-nbt without a dependency on adventure-api.

The current state of things defines the bare minimum core of the system, with some example conversions in the Gson serializer.

Thing still left to consider:

• Updating Gson tests for changed output (do we write test cases to target a specific version?)
• Move feature flag logic into common json serializer?
• How do we take advantage of feature flags in adventure-nbt to toggle feat(nbt): Implement nameless binary serialization #968 (will be handled in its own PR)
• Is protocol version the best way to version our output?
• Finish up test coverage