worker: leave TODO comments for using std::variant when possible

We don’t use C++17 as a baseline yet, but once we do, we can use
`std::variant` to make a few things more performant (and cleaner).
There is a similar TODO comment in the crypto source already.

PR-URL: https://github.com/nodejs/node/pull/38788


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>