wit-parser: change serde format for enum Stability

[ydnar]

This PR makes the JSON representation of enum Stability consistent with other enums in this crate, using the externally tagged representation: https://serde.rs/enum-representations.html#externally-tagged

Originally proposed here: https://bytecodealliance.zulipchat.com/#narrow/stream/327223-wit-bindgen/topic/wit-parser.20crate.3A.20JSON.20format.20changes/near/443516048

Before:

"stability": {
  "type": "unstable",
  "feature": "active"
}

After:

"stability": {
  "unstable": {
    "feature": "active"
  }
}