Merge upstream spec/main into threads

The upstream spec repo hasn't been merged in quite some time (#207 in 2023 may have been the last time AFAICT). We'd like to fork from this repo for the relaxed atomics proposal (which is to be split out from the shared-everything-threads proposal), but to do so we'd like to have this repo be up-to-date first.

Can we merge the latest upstream spec repo into this repo? Also related is #240 and #222.