Should we drop support for testing under d8?

We have spotty test coverage for running emscripten output under d8.

I spend some time recently improving things which culminated in this PR which attempt to run the entire core suite under d8: https://github.com/emscripten-core/emscripten/pull/18837.

While working on this, I realized that maybe we should just drop support completely, simplifying our code and our testing matrix.

My two main reasons for this thinking are:

1. In the past we have relied on direct testing under d8 in order to test with latest features, but as of #18705 we now have a way to test against node canary which uses the very latest version from d8.
2. There are some things that d8 doesn't seem to support which makes getting full coverage hard. For example unhandled promise rejections don't seem to be reported.