refactor: Consistently call HMR messages `message` (#83341)
The HMR messages we send through the WebSocket to the browser were
inconsistently called `message`, `msg`, `obj`, `payload`, or `action`.
With this PR, we're streamlining the naming to consistently use
`message` throughout the codebase. In addition, the discriminating
property `action` is renamed to `type`, aligning it with Turbopack's own
HMR messages.
As an alternative, I considered naming them `action` instead, but this
did not yield the desired consistency across the codebase. Ultimately, I
decided to stick with `message` to avoid confusion and maintain
alignment with Turbopack's terminology (see
`@vercel/turbopack-ecmascript-runtime/browser/dev/hmr-client/hmr-client`).
