worker: make MessagePort `uv_async_t` inline field

It’s not obvious why this was a heap allocation in the first place,
but it’s unneccessary. Most other `HandleWrap`s also store the
libuv handle directly.

PR-URL: https://github.com/nodejs/node/pull/26271


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>