Added Ping/Pong RPC message circuit for Rust <-> Js plugin interaction

[alshdavid]

↪️ Pull Request

• Tidied up the ParcelNapi constructor a little
• Moved Parcel (within ParcelNapi) into its own thread
  • Interactions with Parcel now occur over a channel
• Added rpc callback to ParcelNapi Js options object
• Added napi to parcel_plugin_rpc behind nodejs feature
• Added Ping/Pong circuit to prove a round trip works
• Added napi bindings to RpcHostNodejs (not an unsafe in sight :rofl:)
• Updated the RpcHostMessage enum to reflect changes
• Updated parcel-v3 test to test the Ping/Pong circuit

🚨 Test instructions

yarn build-native && yarn test:integration

✔️ PR Todo

• [x] Added/updated unit tests for this change
• [x] Filled out test instructions (In case there aren't any unit tests)
• [x] Included links to related issues/PRs