doc: move "Core Promise APIs" to "Completed initiatives"

[aduh95]

There haven't been much more progress, it seems safe to assume what could realistically be done has been done.

Closes: nodejs/TSC#1094

[nodejs-github-bot]

Review requested:

• @nodejs/tsc

[ljharb]

it'd be nice to get child_process done before calling it "completed"; i've had it on my list to do but haven't made time yet.

[jasnell]

@ljharb ... there's really nothing preventing continued work. This is really just to remove it from the brief review we have of these during the TSC meetings. This was initially added as a strategic initiative many years ago when it was still controversial that we would adopt promises widely at all. Since then it's been a steady stream of "no updates" reports in the TSC meetings that don't really have any value.

[joyeecheung]

Note that it can be brought back to the ongoing list again if you have work that you want to report/get feedback from the TSC for a sustained period (for one-off TSC intervention requests, tagging tsc-agenda would suffice. Though I don't expect TSC oversight to be necessary for work in this line). @ljharb

[ljharb]

ah ok, gotcha. i'll keep it on my list then :-) thanks!

[mcollina]

lgtm

[aduh95]

@ljharb yeah you're most welcome to keep working on it, just so you know a satisfying async API would probably require some changes at the libuv level – JS only solutions tend to struggle to get traction/consensus, e.g. #45774

[LiviaMedeiros]

LGTM; this might need s/more/move/ in the commit message

[aduh95]

Landed in 19876f4