Detect when Stage 3+ ES proposals get merged into main spec

[dontcallmedom]

Right now, our workflows determine when an ES spec reaches stage 3, but no signal is detected when a spec reaches stage 4 and thus should no longer be listed on its own in browser-specs.

[dontcallmedom]

Some elements of research on this:

• the list of Stage 4 proposals is at https://github.com/tc39/proposals/blob/master/finished-proposals.md
• being stage 4 is not strictly equivalent to being merged into the main spec apparently - transitioning to stage 4 is the start of that process, but they're not simultaneous. For instance, https://tc39.es/proposal-top-level-await/ has reached stage 4, but hasn't been merged in the main spec yet

So, a good process for this may be:

• whenever a browser-specs tracked proposal is detected in the stage 4 list, create a draft pull request that removes it from the main list
• in that pull request, request to add a link to the upstream pull request in ecma262 where we can track progress on the merge

[tidoust]

Could there be stage 3 proposals that get dropped after all?

On top of (or instead of?) looking at stage 4 proposals, it could make sense to compare the stage 3 list with the current list of TC39 proposals in browser-specs.