fix: require serialization for HMRConnection.send on implementation side

[hi-ogawa]

Description

This is brought up by @jamesopstad (Also thanks for spotting the bug on my HMRConnection implementation!).

Currently HMRConnection.send and HMRConnection.onUpdate interfaces are unbalanced as HMRConnection.send: (message: string) => void receives already serialized string of HotPayload. Custom webSocket based implementation would require something like this:
https://github.com/hi-ogawa/vite-environment-examples/blob/edf04cf23fb472a9d085e1a424ae7543164b98f8/packages/workerd/src/worker.ts#L95-L107

      hmr: {
        connection: {
          isReady: () => true,
          onUpdate(callback: (payload: HotPayload) => void) {
            webSocket.addEventListener("message", (event) => {
              callback(JSON.parse(event.data));
            });
          },
          send(messages: string) {
            webSocket.send(messages);
          },
        },
      },

This PR changes it to HMRConnection.send: (payload: HotPayload) => void, so HMRConnection implementation will become like below:

      hmr: {
        connection: {
          ...
          send(payload: HotPayload) {
            webSocket.send(JSON.stringify(payload));
          },
        },
      },

I think this will make interfaces more uniform since HotChannel on server side also assumes (de)serialization to be taken care by implementations. Also in case of ServerHMRConnector / ServerHotChannel, this removes unnecessary serialization since they can pass payload object directly.

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[sheremet-va]

Looks good to me! Funny how we missed it 😄

[hi-ogawa]

Also I remembered this one was saying the opposite, so updated it as well.

[jamesopstad]

Thanks for making these changes. Looks great to me!