PPX: Support open variants

Sometimes there are type definitions like this:

type status = [> `Foo | `Bar ]

let handle_status (status : status) : string =
  match status with
  | `Foo -> "It's Foo!"
  | `Bar -> "It's Bar!"
  | _ -> "Unknown status."

This is useful because this code is "future compatible", meaning a server could add new cases without the client crashing, or the other way around.

Would be nice to support it in melange-json PPX.

Start by locating the melange-json PPX entry points and the existing handling for variant type definitions. Use the open-variant example in this issue to understand the expected behavior, then inspect related tests or add coverage for known and unknown constructors. Done means the PPX supports encoding and decoding the open variant without breaking the fallback case.